Ultrasound cavitation and exfoliation dynamics of 2D materials re-vealed in operando by X-ray free electron laser megahertz imaging

Ultrasonic liquid phase exfoliation is a promising method for the production of two-dimensional (2D) layered materials. A large number of studies have been made in investigating the underlying ultrasound exfoliation mechanisms. However, due to the experimental challenges for capturing the highly transient and dynamic phenomena in real-time at sub-microsecond time and micrometer length scales simultaneously, most theories reported to date still remain elusive. Here, using the ultra-short X-ray Free Electron Laser pulses (~25ps) with a unique pulse train structure, we applied MHz X-ray Microscopy and machine-learning technique to reveal unambiguously the full cycles of the ultrasound cavitation and graphite layer exfoliation dynamics with sub-microsecond and micrometer resolution. Cyclic fatigue shock wave impacts produced by ultrasound cloud implosion were identified as the dominant mechanism to deflect and exfoliate graphite layers mechanically. For the graphite flakes, exfoliation rate as high as ~5 angstroms per shock wave impact was observed. For the HOPG graphite, the highest exfoliation rate was ~0.15 angstroms per impact. These new findings are scientifically and technologically important for developing industrial upscaling strategies for ultrasonic exfoliation of 2D materials

First operation of the JUNGFRAU detector in 16-memory cell mode at European XFEL

The JUNGFRAU detector is a well-established hybrid pixel detector developed at the Paul Scherrer Institut (PSI) designed for free-electron laser (FEL) applications. JUNGFRAU features a charge-integrating dynamic gain switching architecture, with three different gain stages and 75 μm pixel pitch. It is widely used at the European X-ray Free-Electron Laser (EuXFEL), a facility which produces high brilliance X-ray pulses at MHz repetition rate in the form of bursts repeating at 10 Hz. In nominal configuration, the detector utilizes only a single memory cell and supports data acquisition up to 2 kHz. This constrains the operation of the detector to a 10 Hz frame rate when combined with the pulsed train structure of the EuXFEL. When configured in so-called burst mode, the JUNGFRAU detector can acquire a series of images into sixteen memory cells at a maximum rate of around 150 kHz. This acquisition scheme is better suited for the time structure of the X-rays as well as the pump laser pulses at the EuXFEL. To ensure confidence in the use of the burst mode at EuXFEL, a wide range of measurements have been performed to characterize the detector, especially to validate the detector alibration procedures. In particular, by analyzing the detector response to varying photon intensity (so called ‘intensity scan’), special attention was given to the characterization of the transitions between gain stages. The detector was operated in both dynamic gain switching and fixed gain modes. Results of these measurements indicate difficulties in the characterization of the detector dynamic gain switching response while operated in burst mode, while no major issues have been found with fixed gain operation. Based on this outcome, fixed gain operation mode with all the memory cells was used during two experiments at EuXFEL, namely in serial femtosecond protein crystallography and Kossel lines measurements. The positive outcome of these two experiments validates the good results previously obtained, and opens the possibility for a wider usage of the detector in burst operation mode, although compromises are needed on the dynamic range

Tuning of colossal dielectric constant in gold-polypyrrole composite nanotubes using in-situ x-ray diffraction techniques

In-situ x-ray diffraction technique has been used to study the growth process of gold incorporated polypyrrole nanotubes that exhibit colossal dielectric constant due to existence of quasi-one-dimensional charge density wave state. These composite nanotubes were formed within nanopores of a polycarbonate membrane by flowing pyrrole monomer from one side and mixture of ferric chloride and chloroauric acid from other side in a sample cell that allows collection of x-ray data during the reaction. The size of the gold nanoparticle embedded in the walls of the nanotubes was found to be dependent on chloroauric acid concentration for nanowires having diameter more than 100 nm. For lower diameter nanotubes the nanoparticle size become independent of chloroauric acid concentration and depends on the diameter of nanotubes only. The result of this study also shows that for 50 nm gold-polypyrrole composite nanotubes obtained with 5.3 mM chloroauric acid gives colossal dielectric constant of about 107. This value remain almost constant over a frequency range from 1Hz to 106 Hz even at 80 K temperature

Tuning of colossal dielectric constant in gold-polypyrrole composite nanotubes using in-situ X-ray diffraction techniques

In-situ X-ray diffraction technique has been used to study the growth process of gold incorporated polypyrrole nanotubes that exhibit colossal dielectric constant due to existence of quasi-one-dimensional charge density wave state. These composite nanotubes were formed within nanopores of a polycarbonate membrane by flowing pyrrole monomer from one side and mixture of ferric chloride and chloroauric acid from other side in a sample cell that allows collection of X-ray data during the reaction. The size of the gold nanoparticle embedded in the walls of the nanotubes was found to be dependent on chloroauric acid concentration for nanowires having diameter more than 100 nm. For lower diameter nanotubes the nanoparticle size become independent of chloroauric acid concentration and depends on the diameter of nanotubes only. The result of this study also shows that for 50 nm gold-polypyrrole composite nanotubes obtained with 5.3 mM chloroauric acid gives colossal dielectric constant of about 107. This value remain almost constant over a frequency range from 1 Hz to 106 Hz even at 80 K temperature

Evidence of the charge-density wave state in polypyrrole nanotubes

We present a detailed investigation of the low-frequency dielectric and conductivity properties of conducting polymer nanowires. Our results, obtained by connecting ∼107 nanowires in parallel, show that these polypyrrole nanowires behave like conventional Charge-density Wave (CDW) materials, in their nonlinear and dynamic response, together with scaling of relaxation time and conductivity. The observed Arrhenius law for both these quantities gives a CDW gap of 3.5 meV in the regime of temperature (∼40 K) in which the CDW state survives. We find good agreement with a theory of weakly pinned CDW, screened by thermally excited carriers across the CDW gap. The identification of polymer nanowires as CDW provides us a model system to investigate charge ordering owing to electrostatic interaction, relevant to a variety of systems from dusty plasma to molecular biology

Phase behaviour and structural properties of two members of biphenylyl benzoate chiral mesogenic series

Phase behaviour and structural properties of two members of biphenylyl benzoate chiral meso-genic series (2F3R and 3F3R) have been investigated. While both the compounds exhibit SmC*phase over a wide temperature range, 2F3R forms orthogonal SmA* from tilted SmC* on heatingbut 3F3R melts directly to the isotropic phase. The SmA* phase of 2F3R is found to have de vriescharacteristics with small effective layer contraction. Both the samples on cooling form hexagonalSmF* phase below SmC* phase. On further cooling soft crystal like hexagonal SmJ* phase isformed in 2F3R, undergoing a change in the tilt direction, but in 3F3R, SmG* phase is formedwithout any change in the tilt direction. A coexistence phase of (SmC*+SmF*) is also observed ina certain temperature range. Slight differences in the dipole moment and molecular conforma-tion of the two molecules give rise to a subtle change in the intermolecular interaction and playan important role in the appearance of different phases in the two compounds. Cell parametersof SmF*, SmG*, SmJ* phases have also been determined. Layer spacings, tilt angles, averageintermolecular spacings and correlation lengths have been measured. How some of theseproperties compare with other members of the series has been discussed

Effect of non-mesogenic chiral terphenylate on the formulation of room temperature ferroelectric liquid crystal mixtures suitable for display applications

By varying the concentration of a chiral terphenylate dopant in a four-component achiral phenyl pyrimidine-based host mixture we have formulated six ferroelectric liquid crystal mixtures. Though the dopant is non-mesogenic it is observed that only 2 wt% of it induces polarity in the host mixture at 32 °C. With increasing concentration, the mixtures show ferroelectric behavior even below ambient temperature down to at least 12 °C. Optical polarizing microscopy, differential scanning calorimetry, dielectric and electro-optic studies reveal that with respect to temperature range, phase sequence, spontaneous polarization, optical tilt angle and switching time the formulated mixtures are suitable for ferroelectric based liquid crystal displays

Effect of silver nanoparticle doping on the physicochemical properties of a room temperature ferroelectric liquid crystal mixture

We have investigated the effect of doping of a small amount of silver nanoparticles (AgNPs) on the physicochemical properties of a room temperature ferroelectric liquid crystal mixture. Polarizing optical microscopy, differential scanning calorimetry, synchrotron X-ray diffraction, dielectric and electro-optic studies showed that the inclusion of AgNPs greatly influences the phase transition temperatures, structural, dielectric, and electro-optic properties. Doping slightly decreases the SmC*-SmA* and SmA*-Iso transition temperatures signifying decreased ordering which is also reflected in reduced tilt angles. The Goldstone mode dielectric increment of the nanocomposite increases twofold but the critical frequency decreases slightly. Spontaneous polarization of the mixture also increases significantly in the presence of AgNPs whereas the conductivity decreases indicating trapping of ions by the nanoparticles. Goldstone mode rotational viscosity decreases and as a result of which the switching time also decreases by 88 μs making the nano doped mixture more suitable for practical applications

Asymmetric water diffusion driven nanotube actuator

Here we report, water vapor driven actuation of polymer nanotubes, embedded in a nanoporous membrane with one end attached to a surface deposited thin polymer layer. The nanotube composite shows oscillatory motion when placed near water. Permeation of water vapor through these nanotube embedded membranes is found to be direction dependent. With the water vapor as the driving force, the actuator can lift a mass 1000 times heavier than itself with a change in relative humidity of less than 40%. The actuation mechanism arises due to efficient absorption of water molecules by polymer nanotubes and their rapid evaporation through the surface deposited polymer layer. This actuator can be used as artificial muscle and the direction dependent water transport may help in understanding the activities of transmembrane channels and pumps of biological cells. With the aid of a nanowire generator, the oscillatory motion can be used to generate electricity too

Hybrid nanotubes: single step formation of homogeneous nanotubes of polypyrrole-gold composites and novel switching transition of resistance beyond liquid nitrogen temperature

Nanowires of polypyrrole have exhibited switching transition that reduces the resistance of the wires by several orders of magnitude under certain bias around and below 30 K temperature. Here, we have shown that by incorporating gold in these polypyrrole nanotubes using a cost effective template based single-step chemical synthesis technique, this novel resistance switching transition could be extended beyond liquid nitrogen temperature (> 90 K) to make this phenomena technologically relevant. The single step synthesis technique, reported here, provides us uniform mixing of gold and polypyrrole during the formation of composite-nanotubes; with appropriate choice of materials, this synthesis technique can be extended to form nanotubes of other metal-polymer composites