Change of Conduction Mechanism in Polymer/Single Wall Carbon Nanotube Composites upon Introduction of Ionic Liquids and Their Investigation by Transient Absorption Spectroscopy: Implication for Thermoelectric Applications

Polymer composites based on polycarbonate (PC) and polyether ether ketone (PEEK) filled with single-walled carbon nanotubes (SWCNTs, 0.5-2.0 wt %) were melt-mixed to investigate their suitability for thermoelectric applications. Both types of polymer composites exhibited positive Seebeck coefficients (S), indicative for p-type thermoelectric materials. As an additive to improve the thermoelectric performance, three different ionic liquids (ILs), specifically THTDPCl, BMIMPF6, and OMIMCl, were added with the aim to change the thermoelectric conduction type of the composites from p-type to n-type. It was found that in both composite types, among the three ILs employed, only the phosphonium-based IL THTDPCl was able to activate the p- to n-type switching. Moreover, it is revealed that for the thermoelectric parameters and performance, the SWCNT:lL ratio plays a role. In the selected systems, S-values between 61.3 μV/K (PEEK/0.75 wt % SWCNT) and -37.1 μV/K (PEEK/0.75 wt % SWCNT + 3 wt % THTDPCl) were reached. In order to shed light on the physical origins of the thermoelectric properties, the PC-based composites were studied using ultrafast laser time-resolved transient absorption spectroscopy (TAS). The TAS studies revealed that the introduction of ILs in the developed PC/CNT composites leads to the formation of biexcitons when compared to the IL-free composites. Moreover, no direct correlation between S and exciton lifetimes was found for the IL-containing composites. Instead, the exciton lifetime decreases while the conductivity seems to increase due to the availability of more free-charge carriers in the polymer matrix.

Liquid Phase Isolation of SnS Monolayers with Enhanced Optoelectronic Properties

Recent advances in atomically thin two dimensional (2D) anisotropic group IV_A -VI metal monochalcogenides (MMCs) and their fascinating intrinsic properties and potential applications are hampered due to an ongoing challenge of monolayer isolation. Among the most promising MMCs, tin (II) sulfide (SnS) is an earth-abundant layered material with tunable bandgap and anisotropic physical properties, which render it extraordinary for electronics and optoelectronics. To date, however, the successful isolation of atomically thin SnS single layers at large quantities has been challenging due to the presence of strong interlayer interactions, attributed to the lone-pair electrons of sulfur. Here, a novel liquid phase exfoliation approach is reported, which enables the overcome of such strong interlayer binding energy. Specifically, it demonstrates that the synergistic action of external thermal energy with the ultrasound energy-induced hydrodynamic force in solution gives rise to the systematic isolation of highly crystalline SnS monolayers (1L-SnS). It is shown that the exfoliated 1L-SnS crystals exhibit high carrier mobility and deep-UV spectral photodetection, featuring a fast carrier response time of 400 ms. At the same time, monolayer-based SnS transistor devices fabricated from solution present a high on/off ratio, complemented with a responsivity of 6.7 × 10^-3 A W^-1 and remarkable stability upon prolonged operation in ambient conditions. This study opens a new avenue for large-scale isolation of highly crystalline SnS and other MMC manolayers for a wide range of applications, including extended area nanoelectronic devices, printed from solution.

Fast and selective reduction of nitroarenes under visible light with an earth-abundant plasmonic photocatalyst

Reduction of nitroaromatics to the corresponding amines is a key process in the fine and bulk chemicals industry to produce polymers, pharmaceuticals, agrochemicals and dyes. However, their effective and selective reduction requires high temperatures and pressurized hydrogen and involves noble metal-based catalysts. Here we report on an earth-abundant, plasmonic nano-photocatalyst, with an excellent reaction rate towards the selective hydrogenation of nitroaromatics. With solar light as the only energy input, the chalcopyrite catalyst operates through the combined action of hot holes and photothermal effects. Ultrafast laser transient absorption and light-induced electron paramagnetic resonance spectroscopies have unveiled the energy matching of the hot holes in the valence band of the catalyst with the frontier orbitals of the hydrogen and electron donor, via a transient coordination intermediate. Consequently, the reusable and sustainable copper-iron-sulfide (CuFeS₂) catalyst delivers previously unattainable turnover frequencies, even in large-scale reactions, while the cost-normalized production rate stands an order of magnitude above the state of the art.

Laser-Induced Erasable and Re-Writable Waveguides within Silver Phosphate Glasses

Femtosecond direct laser writing is a well-established and robust technique for the fabrication of photonic structures. Herein, we report on the fabrication of buried waveguides in AgPO₃ silver metaphosphate glasses, as well as, on the erase and re-writing of those structures, by means of a single femtosecond laser source. Based on the fabrication procedure, the developed waveguides can be erased and readily re-inscribed upon further femtosecond irradiation under controlled conditions. Namely, for the initial waveguide writing the employed laser irradiation power was 2 J/cm² with a scanning speed of 5 mm/s and a repetition rate of 200 kHz. Upon enhancing the power to 16 J/cm² while keeping constant the scanning speed and reducing the repetition rate to 25 kHz, the so formed patterns were readily erased. Then, upon using a laser power of 2 J/cm² with a scanning speed of 1 mm/s and a repetition rate of 200 kHz the waveguide patterns were re-written inside the glass. Scanning electron microscopy (SEM) images at the cross-section of the processed glasses, combined with spatial Raman analysis revealed that the developed write/erase/re-write cycle, does not cause any structural modification to the phosphate network, rendering the fabrication process feasible for reversible optoelectronic applications. Namely, it is proposed that this non-ablative phenomenon lies on the local relaxation of the glass network caused by the heat deposited upon pulsed laser irradiation. The resulted waveguide patterns Our findings pave the way towards new photonic applications involving infinite cycles of write/erase/re-write processes without the need of intermediate steps of typical thermal annealing treatments.

Advanced composite glasses with metallic, perovskite, and two-dimensional nanocrystals for optoelectronic and photonic applications

This article reviews the tremendous advancement of the optoelectronic and photonic properties of inorganic oxide glasses upon the incorporation of metallic, perovskite, and two-dimensional nanocrystals within their matrix. In the first part, we present the exploitation of typical inorganic oxide glasses as hosting platforms for the incorporation of metallic nanoparticles. Such a method offers tremendous advantages in terms of inducing plasmonic features that enable the tunability of the photonic properties of the embedded materials. Along similar lines, due to their exceptional photoluminescence properties all inorganic lead halide perovskites show enormous potential for next generation light-emitting, optoelectronic and photonic devices. To date, however, their usage is limited significantly by their poor chemical stability upon exposure to moisture, and lead toxicity issues. A recent and highly promising approach for overcoming these important challenges is the encapsulation of perovskite nanocrystals within inorganic oxide glasses. Based on this, in the second section we focus on the recent advancements in perovskite glasses in terms of the developed fabrication procedures and the resulting optoelectronic features, while considering the production limitations. In the last part, we consider the development of composite two-dimensional materials glass architectures in terms of the available synthesis routes and the novelty of their optical and emission features. Finally, future perspectives on the described composite glass systems in terms of potential applications are summarized.

Robust B-exciton emission at room temperature in few-layers of MoS2:Ag nanoheterojunctions embedded into a glass matrix

Tailoring the photoluminescence (PL) properties in two-dimensional (2D) molybdenum disulfide (MoS2) crystals using external factors is critical for its use in valleytronic, nanophotonic and optoelectronic applications. Although significant effort has been devoted towards enhancing or manipulating the excitonic emission in MoS2 monolayers, the excitonic emission in few-layers MoS2 has been largely unexplored. Here, we put forward a novel nano-heterojunction system, prepared with a non-lithographic process, to enhance and control such emission. It is based on the incorporation of few-layers MoS2 into a plasmonic silver metaphosphate glass (AgPO3) matrix. It is shown that, apart from the enhancement of the emission of both A- and B-excitons, the B-excitonic emission dominates the PL intensity. In particular, we observe an almost six-fold enhancement of the B-exciton emission, compared to control MoS2 samples. This enhanced PL at room temperature is attributed to an enhanced exciton-plasmon coupling and it is supported by ultrafast time-resolved spectroscopy that reveals plasmon-enhanced electron transfer that takes place in Ag nanoparticles-MoS2 nanoheterojunctions. Our results provide a great avenue to tailor the emission properties of few-layers MoS2, which could find application in emerging valleytronic devices working with B excitons.

Highly luminescent and ultrastable cesium lead bromide perovskite patterns generated in phosphate glass matrices

Owing to their exceptional optoelectronic properties, all-inorganic lead halide perovskites offer enormous potential for next generation photonic, light-emitting, and optoelectronic devices. However, their usage is significantly limited by their poor stability upon moisture exposure and lead toxicity issues. Moreover, many of the aforementioned applications rely on the development of confined perovskite patterns of various shapes and periodicities. Here we report a simple and low-temperature method enabling the controlled incorporation of photoluminescent all-inorganic metal halide PNCs into a silver phosphate glass (AgPO3) matrix which is transparent in most of the visible range. The developed fabrication protocol is based on a simple melting encapsulation process in which pre-synthesized perovskite crystals are inserted in the glass matrix, following the initial glass quenching. Using this novel approach, two types of composite perovskite glasses are prepared, one that hosts perovskite isles and the second in which a thin perovskite layer is embedded beneath the glass surface. Both types of composite glasses exhibit remarkable photoluminescence stability when compared to the ambient air-exposed perovskite crystals. More importantly, by means of a simple and fast cw-laser processing technique, we demonstrate the development of encapsulated dotted perovskite micropatterns within the composite perovskite glass. The ability of the proposed system to resolve stability and lead toxicity issues, coupled with the facile formation of highly luminescent perovskite patterns pave the way towards the broad exploitation of perovskite crystals in photonic applications.

Limitations of a polymer-based hole transporting layer for application in planar inverted perovskite solar cells

Planar inverted lead halide photovoltaics demonstrate remarkable photoconversion properties when employing poly(triarylamine) (PTAA) as a hole transporting layer. Herein, we elucidate the effect of ambient ultraviolet (UV) degradation on the structural and operational stability of the PTAA hole transporter through a series of rigorous optoelectrical characterization protocols. Due attention was given to the interplay between the polymer and perovskite absorber, both within the framework of a bilayer structure and fully assembled solar cells. The obtained results imply that UV degradation exerts a major influence on the structural integrity of PTAA, rather than on the interface with the perovskite light harvester. Moreover, UV exposure induced more adverse effects on tested samples than environmental humidity and oxygen, contributing more to the overall reduction of charge extraction properties of PTAA, as well as increased defect population upon prolonged UV exposure.

Enhancement of the Power-Conversion Efficiency of Organic Solar Cells via Unveiling an Appropriate Rational Design Strategy in Indacenodithiophene- alt-quinoxaline π-Conjugated Polymers

We report on the photovoltaic parameters, photophysical properties, optoelectronic properties, self-assembly, and morphology variations in a series of high-performance donor-acceptor (D-A) π-conjugated polymers based on indacenodithiophene and quinoxaline moieties as a function of the number-average molecular weight ([Formula: see text]), the nature of aryl substituents, and the enlargement of the polymer backbone. One of the most important outcome is that from the three optimization approaches followed to tune the chemical structure toward enhanced photovoltaic performance in bulk heterojunction solar cell devices with the fullerene derivative [6,6]-phenyl-C₇₁-butyric acid methyl ester as the electron acceptor, the choice of the aryl substituent is the most efficient rational design strategy. Incorporation of thienyl rings as substituents versus phenyl rings accelerates the electron-hole extraction process to the respective electrode, despite the slightly lower recombination lifetime and, thus, improves the electrical performance of the device. Single-junction solar cells based on ThIDT-TQxT feature a maximum power-conversion efficiency of 7.26%. This study provides significant insights toward understanding of the structure-properties-performance relationship for D-A π-conjugated polymers in solid state, which provide helpful inputs for the design of next-generation polymeric semiconductors for organic solar cells with enhanced performance.

Bioresorbable optical fiber Bragg gratings

We demonstrate, for the first time, an inscription and wet dissolution study of Bragg gratings in a bioresorbable calcium-phosphate glass optical fiber. Bragg gratings, with average refractive index changes of 5.8×10^-4, were inscribed using 193 nm excimer laser radiation. Results on the dissolution of the irradiated fiber in simulated physiological conditions are presented after immersing a tilted Bragg grating in a phosphate buffered saline solution for 56 h; selective chemical etching effects are also reported. The investigations performed pave the way toward the use of such phosphate glass fiber Bragg gratings for the development of soluble photonic sensing probes for the efficient in vivo monitoring of vital mechanical or chemical parameters.

Improved Carrier Transport in Perovskite Solar Cells Probed by Femtosecond Transient Absorption Spectroscopy

CH₃NH₃PbI₃ perovskite thin films have been deposited on glass/indium tin oxide/hole transport layer (HTL) substrates, utilizing two different materials as the HTLs. In the first configuration, the super hydrophilic polymer poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), known as PEDOT:PSS, was employed as the HTL material, whereas in the second case, the nonwetting poly(triarylamine) semiconductor polymer, known as PTAA, was used. It was found that when PTAA is used as the HTL material, the averaged power conversion efficiency (PCE) of the perovskite solar cells (PSCs) remarkably increases from 12.60 to 15.67%. To explore the mechanism behind this enhancement, the aforementioned perovskite/HTL arrangements were investigated by time-resolved transient absorption spectroscopy (TAS) performed under inert conditions. By means of TAS, the charge transfer, carrier trapping, and hole injection dynamics from the photoexcited perovskite layers to the HTL can be directly monitored via the characteristic bleaching profile of the perovskite at ∼750 nm. TAS studies revealed faster relaxation times and decay dynamics when the PTAA polymer is employed, which potentially account for the enhanced PCE observed. The TAS results are correlated with the structure and crystalline quality of the corresponding perovskite films, investigated by scanning electron microscopy, X-ray diffraction, atomic force microscopy, micro-photoluminescence, and transmittance spectroscopy. It is concluded that TAS is a benchmark technique for the understanding of the carrier transport mechanisms in PSCs and constitutes a figure-of-merit tool toward their efficiency improvement.

Silver iodide phosphate glass microsphere resonator integrated on an optical fiber taper

In this Letter, we demonstrate the fabrication and characterization of a robust and functional whispering gallery mode (WGM) resonating system based on a silver iodide phosphate glass microsphere melted on an optical fiber taper. The fabrication process is presented, together with spectral characterization of the device. The effect of the thermal annealing of the soft glass resonator on the whispering gallery modes' excitation and Q-factor is shown and discussed.

Vibrational spectroscopic and bond valence study of structure and bonding in Al2O3-containing AgI–AgPO3 glasses

We present a detailed investigation of the effects of synthesis conditions on glasses xAgI–(1 − x)AgPO₃ with 0 ≤ x ≤ 0.4.

Light driven optofluidic switch developed in a ZnO-overlaid microstructured optical fiber

A great challenge of Optofluidics remains the control of the fluidic properties of a photonic circuit by solely utilizing light. In this study, the development of a ZnO nanolayered microstructured optical fiber (MOF) Fabry-Perot interferometer is demonstrated, along with its fully reversible optofluidic switching behaviour. The actuation and switching principle is entirely based on the employment of light sources, i.e. UV 248 nm and green 532 nm lasers, while using modest irradiation doses. The synthesized ZnO within the MOF capillaries acts as a light triggered wettability transducer, allowing the controlled water filling and draining of the MOF Fabry-Perot cavity. The progression of the optofluidic cycle is monitored in situ with optical microscopy, while Fabry-Perot reflection spectra are monitored in real time to probe temporal infiltration behaviour. Finally, a first insight on the light triggered switching mechanism, employing photoluminescence and spectrophotometric measurements is presented. Results appear highly promising towards the design of smart in-fiber optofluidic light switching devices, suitable for actuating and sensing applications.

Silver plasmon resonance effects in AgPO3/silica photonic bandgap fiber

We report on the application of an external electric field and its tuning effect in the guiding properties of a composite AgPO3/silica photonic bandgap fiber. The application of an electric field leads to the poling of the soft glass inclusion, resulting in the formation of a silver-induced plasmonic band, predominantly affecting the short wavelength guiding regimes while inducing polarization dependent losses. These spectral effects are attributed to the formation of silver nanoparticles within the AgPO3 glass matrix, driven by thermal poling.

Photosensitive, all-glass AgPO3/silicaphotonic bandgap fiber

Photonic bandgap (PBG) guidance is observed in a solid core photonic crystal fiber (PCF) consisting of silver metaphosphate (AgPO(3)) glass embedded into a silica cladding, realized by vacuum-assisted infiltration of the molten glass into the hollow channels of a commercial silica PCF. Morphologic analysis of the cladding microstructure by optical and scanning electron microscopy reveals the formation of highly homogeneous glass strands along the PCF length. The characteristic transmission spectrum of the fiber shows PBG guidance in the range between 350 and 1650 nm. The exposure of the cladding glass matrix, using 355 nm, 150 ps laser irradiation, allows photo-induced enhancement of the transmission-to-stop-band extinction ratio by ∼60 dB/cm and bandwidth tuning. Numerical calculations of the transmission pattern of the fabricated AgPO(3)/silica bandgap fiber are in good agreement with experiments.

A Mechanistic Approach to Conductivity Relaxation in Ionic Glasses

A mechanism is proposed for conductivity relaxation in ionic glasses, based on the operation of a memory effect (as postulated in the dynamic structure model) that provides the necessary synergy for ‘pair-wise’ coupling of ionic motions in single cation glasses. It is suggested that this mechanism (which resembles in some respects the older ‘interstitialcy’ process) constitutes an essential component of the single and many-particle reaction route’ in Funke’s concept of mismatch and relaxation (CMR). This coupling is also present in mixed cation glasses, but the appearance of a site mismatch effect and the consequent disappearance of synergy leads to anomalous behaviours typical of the mixed cation effect, including recently reported increases in activation volume.

What variable-pressure variable-temperature measurements are telling us about ion transport in glass

Variable-pressure, variable-temperature (VPVT) measurements of ionic conductivity in typical inorganic (sodium aluminoborate and lithium-sodium phosphate) glasses provide information concerning two independent activation parameters: the activation energy (E(A) = - R d ln sigma T/d(1/T)) and the activation volume (V(A) = - RT d ln sigma/dP). In single-cation glasses there is no simple link between E(A) and V(A). The strong increase in E(A) seen with decreasing alkali content is not paralleled by an increase in V(A). However, in mixed cation glasses, maxima are observed in both E(A) and V(A). To account for these observations, a new model is being developed where ion transport is limited by a shortage of available volume. In this model, the measured V(A) values represent the volumes of opened up 'target sites', the larger values observed in mixed-cation glasses providing evidence for the coupled motion of unlike ions. The extent of this pairwise coupling increases with increasing temperature.