We examine the magnetic properties of ∼23 nm single domain nanocubes and ∼200 nm multidomain iron oxide nanoparticles that are surface functionalized with poly(L- or D-phenylalanine) chiral brushes of variable length. Interestingly, the larger nanoparticles manifest a remanent magnetization in all directions, i.e., display monopole-like or hedgehog behaviour that depends on the handedness of the brush. Conversely, no such response is observed in the smaller nanoparticles. The emergent monopole-like magnetic properties are attributed to the chiral-induced spin selectivity effect acting on the magnetic domain structure, single vs. multidomain, to imprint a magnetization bias on the nanoparticles. Collectively, this study reveals a facile approach for the formation of hedgehog magnetic nanoparticles and outlines features necessary for their formation.

The dynamics of excited electronic states in self-assembled structures formed between silver-(I) ions and cytosine-containing DNA strands or monomeric cytosine derivatives were investigated by time-resolved infrared (TRIR) spectroscopy and quantum mechanical calculations. The steady-state and time-resolved spectra depend sensitively on the underlying structures, which change with pH and the nucleobase and silver ion concentrations. At pH similar to 4 and low dC(20) strand concentration, an intramolecularly folded i-motif is observed, in which protons, and not silver ions, mediate C-C base pairing. However, at the higher strand concentrations used in the TRIR measurements, dC(20) strands associate pairwise to yield duplex structures containing C-Ag+-C base pairs with a high degree of propeller twisting. UV excitation of the silver ion-mediated duplex produces a long-lived excited state, which we assign to a triplet excimer state localized on a pair of stacked cytosines. The computational results indicate that the propeller-twisted motifs induced by metal-ion binding are responsible for the enhanced intersystem crossing that populates the triplet state and not a generic heavy atom effect. Although triplet excimer states have been discussed frequently as intermediates in the formation of cyclobutane pyrimidine dimers, we find neither computational nor experimental evidence for cytosine-cytosine photoproduct formation in the systems studied. These findings provide a rare demonstration of a long-lived triplet excited state that is formed in a significant yield in a DNA duplex, demonstrating that supramolecular structural changes induced by metal ion binding profoundly affect DNA photophysics.

Despite a few recent reports on Rashba effects in two-dimensional (2D) Ruddlesden-Popper (RP) hybrid perovskites, the precise role of organic spacer cations in influencing Rashba band splitting remains unclear. Here, using a combination of temperature-dependent two-photon photoluminescence (2PPL) and time-resolved photoluminescence spectroscopy, alongside density functional theory (DFT) calculations, we contribute to significant insights into the Rashba band splitting found for 2D RP hybrid perovskites. The results demonstrate that the polarity of the organic spacer cation is crucial in inducing structural distortions that lead to Rashba-type band splitting. Our investigations show that the intricate details of the Rashba band splitting occur for organic cations with low polarity but not for more polar ones. Furthermore, we have observed stronger exciton-phonon interactions due to the Rashba-type band splitting effect. These findings clarify the importance of selecting appropriate organic spacer cations to manipulate the electronic properties of 2D perovskites.

In recent years, there has been an impressively fast technological progress in the development of highly efficient lead halide perovskite solar cells. Nonetheless, the stability of perovskite films and associated solar cells remains a source of uncertainty and necessitates sophisticated characterization techniques. Here, we report low- to mid-frequency resonant Raman spectra of formamidinium-based lead mixed-halide perovskites. The assignment of the different Raman lines in the measured spectra is assisted by DFT simulations of the Raman spectra of suitable periodic model systems. An important result of this work is that both experiment and theory point to an increase of the stability of the perovskite structure with increasing chloride doping concentration. In the Raman spectra, this is reflected by the appearance of new lines due to the formation of hydrogen bonds. Thus, higher chloride doping results in less torsional motion and lower asymmetric bending contributing to higher stability. This study yields a solid basis for the interpretation of the Raman spectra of formamidinium-based mixed-halide perovskites, furthering the understanding of the properties of these materials, which is essential for their full exploitation in solar cells.

Over the past two decades, intensive research efforts have been devoted to suppressions of Auger recombination in metal-chalcogenide and perovskite nanocrystals (PNCs) for the application of photovoltaics and light emitting devices (LEDs). Here, we have explored dodecahedron cesium lead bromide perovskite nanocrystals (DNCs), which show slower Auger recombination time compared to hexahedron nanocrystals (HNCs). We investigate many-body interactions that are manifested under high excitation flux density in both NCs using ultrafast spectroscopic pump-probe measurements. We demonstrate that the Auger recombination rate due to multiexciton recombinations are lower in DNCs than in HNCs. At low and intermediate excitation density, the majority of carriers recombine through biexcitonic recombination. However, at high excitation density (>1018 cm-3) a higher number of many-body Auger process dominates over biexcitonic recombination. Compared to HNCs, high PLQY and slower Auger recombinations in DNCs are likely to be significant for the fabrication of highly efficient perovskite-based photonics and LEDs.

The high optical cross sections of plasmonic nanoparticles make them attractive for applications in photovoltaics and photocatalysis that seek to convert light into electrical charges. However, the ultrafast deactivation of hot electrons via electron-electron scattering has limited their utilization. Here, we describe hot electron transfer (HET) in core-shell nanoparticles with a gold core and a cerium oxide (CeO2) shell and in a three-component hierarchical material in which the core-shell nanoparticles are paired with sheets of reduced graphene oxide (rGO). In particular, femtosecond transient absorption measurements conducted at different excitation wavelengths from the UV to the NIR provide evidence of interband- and intraband-induced HET from gold to CeO2. The quantum yield for the HET to the metal oxide is estimated to be 17% for excitation at 265 nm. The lifetime of almost 3 ns for the transferred electron is longer than has been observed previously in analogous gold-TiO2 hybrid nanostructures. The increased lifetime in gold@CeO2 is explained by electron transfer to localized 4f states in CeO2, which lie between the valence band and the Ce 4f band. Initial results demonstrating the photocatalytic degradation of a dye molecule with our hybrid system illustrate its potential for photocatalytic applications.

Quasi-two-dimensional (2D) perovskites have attracted extraordinary attention for next-generation lighting and displays because of their high color purity and performance. Here, we present the results of an investigation of the distribution of n- phases and their impact on photophysical properties of quasi-2D methylammonium lead bromide perovskite film (q-MPB) where n represents the number of PbBr4 octahedral layers. We find that the emission from the low-n-phase region is blue-shifted compared to the emission from the medium-n-phase and high-n-phase regions. The yield and lifetime of the emission are higher for the medium-n-phase region due to the higher quasi-2D nature of the particles. Temperature-dependent two-photon photoluminescence measurements show that exciton-phonon interaction is stronger for the low-n-phase region, and it decreases when the n-phases increase. Our work demonstrates the impact of spatial heterogeneity of n-phases on light-emission properties, which is of considerable importance for the development of highly efficient next-generation q-MPB-based LEDs.

Here, we quantify the effect of an external magnetic field (β) on the oxygen evolution reaction (OER) for a cobalt oxide|fluorine-doped tin oxide coated glass (CoOx|FTO) anode. A bespoke apparatus enables us to precisely determine the relationship between magnetic flux density (β) and OER activity at the surface of a CoOx|FTO anode. The apparatus includes a strong NdFeB magnet (βmax = 450 ± 1 mT) capable of producing a magnetic field of 371 ± 1 mT at the surface of the anode. The distance between the magnet and the anode surface is controlled by a linear actuator, enabling submillimeter distance positioning of the magnet relative to the anode surface. We couple this apparatus with a finite element analysis magnetic model that was validated by Hall probe measurements to determine the value of β at the anode surface. At the largest tested magnetic field strength of β = 371 ± 1 mT, a 4.7% increase in current at 1.5 V vs the normal hydrogen electrode (NHE) and a change in the Tafel slope of 14.5 mV/dec were observed. We demonstrate through a series of OER measurements at sequential values of β that the enhancement consists of two distinct regions. The possible use of this effect to improve the energy efficiency of commercial water electrolyzers is discussed, and major challenges pertaining to the accurate measurement of the phenomenon are demonstrated.

Two-dimensional (2D) materials have attracted tremendous research interest because of their unique optical, electronic, and mechanical properties which make them suitable building blocks for various electronic and optoelectronic applications. The heterostructures formed by 2D semiconductors can play an important role in modern semiconductor industry. Here, we have grown 2D-CdSe structures over MoS2 nanosheets via successive ion layer adsorption and reaction method. The dynamics of photoinduced charge carriers in 2D-CdSe nanostructures have been investigated using femtosecond transient absorption spectroscopy. Our photophysical studies infer that charge carrier recombination is monomolecular at low and bimolecular at high excitation densities. Nonetheless, femtosecond Z-scan technique was employed to demonstrate optical limiting behavior of 2D-CdSe films. This study will open up a new avenue of designing 2D nanomaterials for optoelectronic applications.

Metal halide-based perovskite semiconductors exhibit excellent optoelectronic properties such as a sharp absorption edge, high absorption coefficients, and a small recombination rate. Mixed compositions result in a variation of the structure of these perovskite materials, which also influences their electronic properties. Even though huge progress in synthesis and device fabrication has been made, still systematic investigations of structural properties of lead halide-based perovskites are missing. Here, we systematically investigate the vibrational features of lead bromide-based perovskites using Raman spectroscopy and density functional theory (DFT). We have performed these investigations using MA(+), FA(+), and Cs+ as cations in the lead bromide structures and determined the vibrational modes both from Raman experiments and DFT simulations. We find a clear dependence of the Raman band wavenumbers on the chosen cations. The structural differences are reflected in the different line-width broadening of Raman bands, charge distribution on the cations and the extent of their interactions with the bromide anions.

The role of intermolecular interactions, in particular those involving organic fluorine, in molecular crystals are of importance. In the current study, six derivatives of benzoyl ferrocene containing fluoro/trifluoromethyl functional groups are synthesized. Their structures have been determined using single crystal X-ray diffraction. The hydrogen bonds in the crystal lattice involving fluorine and oxygen atoms as acceptors are of significance. A quantitative assessment of the contribution of various intermolecular interactions via Hirshfeld surface analysis has been performed and the nature of the contributing interactions, as a cooperative interplay of electrostatics and dispersion interactions has been established via inputs from energy frameworks. It is observed that there exists a relationship between the acidity of the participating hydrogens and the nature and associated energetics of the intermolecular interactions of the type C-H center dot center dot center dot O and C-H center dot center dot center dot F which contribute towards the overall packing of molecules in the solid state. (C) 2020 Elsevier B.V. All rights reserved.

Since the discovery, chiral induced spin selectivity effect or CISS effect, which refers to the preferential transmission of electron of one spin over another through a chiral molecule/material, has generated significant interest due to its versatile application in different branches of science. Over the years, a large number of theoretical and experimental studies are done to understand the mechanism of CISS effect and also to utilize the effect in different field of science; such as memory devices, electrocatalytic experiments, chiral separation etc. In this dissertation, fundamental studies of the chiral molecule at the ferromagnetic interface are investigated through surface potential and magnetic measurements. Experiments are also done to study the spin filtering properties of chiral thin film and explore their electrocatalytic activity In the first study chiral molecule induced magnetization of the superparamagnetic particles was shown. This experiment shows a novel method to create ferromagnetic particle of 10 nm size from superparamagnetic particle using CISS. In the second study, spin dependent charge penetration to chiral molecule from the ferromagnetic substrate is presented by measuring surface potential using Kelvin probe method. In the third study, spin dependent electron transmission through chiral cobalt oxide thin film was performed by using magnetic conductive AFM and magnetoresistance measurements. In the forth study the electrocatalytic efficiency for water splitting was studied using chiral and magnetic thin cobalt oxide film. The finding of these studies serve to progress the CISS field and may help developing new CISS based spintronics device and electrocatalysts.

In recent years, there has been a surge in research efforts to utilize organic-inorganic lead halide perovskites (OLHPs) in optoelectronic devices (e.g., concentrator photovoltaics) requiring high light illumination. Yet, knowledge of the physics of photocarriers in perovskites in the high-excitation regime is limited. Here, we investigate carrier and exciton dynamics that are manifested under strong light illumination in methyl ammonium lead bromide perovskite nanocrystals (NCs) using ultrafast pump-probe spectroscopy. We demonstrate that the carrier trapping process is highly excitation intensity dependent because of a potential barrier that isolates trap states from the band edge. At low excitation densities, holes are unable to cross the energy barrier, but holes undergo trapping at moderate densities (10(18)-10(20) cm(-3)). The charge carrier trapping is negligible due to the dominance of higher-order Auger processes at high excitation density (>10(20) m(-3)). We suggest that the effect of trap states is likely to be insignificant for perovskite concentrator solar cells.

Quasi two-dimensional perovskites have attracted great attention for applications in light-emitting devices and photovoltaics due to their robustness and tunable highly efficient photoluminescence (PL). However, the mechanism of intrinsic PL in these materials is still not fully understood. In this work, we have analysed the nature of the different emissive states and the impact of temperature on the emissions in quasi two-dimensional methyl ammonium lead bromide perovskite (q-MPB) and cesium lead bromide perovskite (q-CPB). We have used spatially resolved phase-modulated two-photon photoluminescence (2PPL) and temperature-dependent 2PPL to characterize the emissions. Our results show that at room temperature, the PL from q-MPB is due to the recombination of excitons and free carriers while the PL from q-CPB is due to the recombination of excitons only. Temperature-dependent measurements show that in both materials the linewidth broadening is due to the interactions between the excitons and optical phonons at high temperatures. Comparing the characteristics of the emissions in the two systems, we conclude that q-CPB is better suited for light emitting devices. With a further optimization to reduce the impact on the environment, q-CPB-based LEDs could perform as well as OLEDs.

Two-dimensional (2D) perovskites with alternating cations in the interlayer space (ACI) represent a new type of structure with different physical properties compared to the more common Ruddlesden–P...

This work explores the use of chiral cobalt oxide thin film electrocatalysts for the oxygen evolution reaction and examines how their spin polarization properties might be used to control electrochemically generated intermediates at different pH values. These studies demonstrate that chiral cobalt oxide electrocatalysts reduce the reaction overpotential by 65 mV at 10 mA/cm2, increase the oxygen yield by 1.4-fold at a fixed current density at pH 10, and decrease the production of hydrogen peroxide by 4.0-fold as compared to that of the corresponding meso-CoOx analogues. Additional studies in which the electrocatalyst is modified to make it paramagnetic exhibit a similar enhancement with an applied external magnetic field. The findings from these studies are described using a proposed mechanistic model, which unifies the favorable effects of chirality and magnetization. The results are attributed to the advantage of spin-polarized intermediates in facilitating the oxygen evolution reaction.

We report the hydrothermally enhanced hydrolysis of polyacrylonitrile (PAN) in neutral water, which generates photoluminescent polymers with low unsaturation degrees. Despite the hydrophobic nature of PAN, the product can be dissolved in water at a high concentration (≥100 g/L). The product exhibits complete absence of alkenes or aromatic structures, and photoluminescence originates from newly formed N- and O-containing groups. The presence of both n to π* and π to π* transitions is confirmed by time-dependent density functional theory (TD-DFT) calculations. The efficient transformation of PAN benefits from the enhanced hydrolysis of nitrile groups. While similar reactions have been reported previously under alkaline environments, we demonstrate that efficient hydrolysis can also occur in neutral water under the hydrothermal condition. Two additional methods based on different mechanisms are discussed to demonstrate the simplicity and efficiency of the hydrothermal reaction.

Kelvin-probe measurements on ferromagnetic thin film electrodes coated with self-assembled monolayers of chiral molecules reveal that the electron penetration from the metal electrode into the chiral molecules depends on the ferromagnet’s magnetization direction and the molecules’ chirality. Electrostatic potential differences as large as 100 mV are observed. These changes arise from the applied oscillating electric field, which drives spin-dependent charge penetration from the ferromagnetic substrate to the chiral molecules. The enantiospecificity of the response is studied as a function of the magnetization strength, the magnetization direction, and the handedness and length of the chiral molecules. These new phenomena are rationalized in terms of the chiral-induced spin selectivity (CISS) effect, in which one spin orientation of electrons from the ferromagnet penetrates more easily into a chiral molecule than does the other orientation. The large potential changes (>kT at room temperature) manifested here imply that this phenomenon is important for spin transport in chiral spintronic devices and for magneto-electrochemistry of chiral molecules.

Organic lead halide perovskite (OLHP) nanocrystals (NCs) have paved the way to advanced optoelectronic devices through their extraordinary electrical and optical properties. However, understanding of the light-induced complex dynamic phenomena in OLHP NCs remains a subject of debate. Here we used wide field microscopy and time-resolved spectroscopy to correlate the local changes in photophysics and the dynamical behavior of photocarriers. We demonstrate that light-induced brightening of the photoluminescence from the formamidinium lead bromide NC films is related to the film preparation condition and reduction of trap density. The density of trap states is reduced via halide ion migration from interstitial position. Our femtosecond transient absorption study identifies transient Stark effect due to the generation of hot carriers. Because of slow carrier trapping, Auger recombination through many-body carrier-carrier interactions dominates over trion recombination. This work presents unprecedented insights into the light-driven processes enabling better device design in the future.