Revisiting a (001)-oriented layered lead chloride templated by 1,2,4-triazolium: structural phase transitions, lattice dynamics and broadband photoluminescence.

This study revisits a (001)-oriented layered lead chloride templated by 1,2,4-triazolium, Tz2PbCl4, which recently has been an object of intense research but still suffers from gaps in characterization. Indeed, the divergent reports on the crystal structures of Tz2PbCl4 at various temperatures, devoid of independent verification of chiral phases through second harmonic generation (SHG), have led to an unresolved debate regarding the existence of a low-temperature phase transition (PT) and the noncentrosymmetric nature of the low-temperature phase. Now, by combining differential scanning calorimetry, single-crystal X-ray diffraction, dielectric, as well as linear and nonlinear optical spectroscopies on Tz2PbCl4, we reveal a sequence of reversible PTs at T1 = 361 K (phase I-II), T2 = 339 K (phase II-III), and T3 = 280 K (phase III-IV). No SHG activity could be registered for any of the four crystal phases, as checked by wide-temperature range SHG screening, supporting their centrosymmetry. The dipole relaxation processes indicate a decrease in activation energy with increasing temperature, from 0.60, 0.38, to 0.24 eV observed for phase IV (space group P21/c), phase III (Pnma), and phase II (Cmcm), respectively. This change is interpreted as a result of the diminishing strength of H-bonds as the system transforms from phase IV to III and subsequently to II. The weaker H-bonds facilitate the reorientation of Tz+ cations in the presence of an external electric field. The photoluminescence spectra of Tz2PbCl4 reveal an intriguing interplay of narrow and broadband emission, linked respectively to free excitons and excitons trapped on defects. Notably, as the temperature decreases from 300 K to 16 K, both the emission bands exhibit distinctive blue and red shifts, indicative of increased in-plane octahedral distortion. This dynamic behaviour transforms the photoluminescence of Tz2PbCl4 from greenish-blue at 300 K to yellowish-green at 13 K, enriching our understanding of 2D lead halide perovskites and highlighting the optoelectronic potential of Tz2PbCl4.

Surface Engineering of Methylammonium Lead Bromide Perovskite Crystals for Enhanced X-ray Detection.

The surface quality of lead halide perovskite crystals can extremely influence their optoelectronic properties and device performance. Here, we report a surface engineering crystallization technique in which we in situ grow a polycrystalline methylammonium lead tribromide (MAPbBr3) film on top of bulk mm-sized single crystals. Such MAPbBr3 crystals with a MAPbBr3 passivating film display intense green emission under UV light. X-ray photoelectron spectroscopy demonstrates that these crystals with emissive surfaces are compositionally different from typical MAPbBr3 crystals that show no emission under UV light. Time-resolved photoluminescence and electrical measurements indicate that the MAPbBr3 film/MAPbBr3 crystals possess less surface defects compared to the bare MAPbBr3 crystals. Therefore, X-ray detectors fabricated using the surface-engineered MAPbBr3 crystals provide an almost 5 times improved sensitivity to X-rays and a more stable baseline drift with respect to the typical MAPbBr3 crystals.

Bandgap Pressure Coefficient of a CH3NH3PbI3 Thin Film Perovskite.

Recent scientific interest in examining the bandgap evolution of a MAPbI3 hybrid perovskite by applying hydrostatic pressure has mostly focused on a room-temperature tetragonal phase. In contrast, the pressure response of a low-temperature orthorhombic phase (OP) of MAPbI3 has not been explored and understood. In this research, we investigate for the first time how hydrostatic pressure alters the electronic landscape of the OP of MAPbI3. Pressure studies using photoluminescence combined with calculations within density functional theory at zero temperature allowed us to identify the main physical factors affecting the bandgap evolution of the OP of MAPbI3. The negative bandgap pressure coefficient was found to be strongly dependent on the temperature (α120K = -13.3 ± 0.1 meV/GPa, α80K = -29.8 ± 0.1 meV/GPa, and α40K = -36.3 ± 0.1 meV/GPa). Such dependence is related to the changes in the Pb-I bond length and geometry in the unit cell as the atomic configuration approaches the phase transition as well as the increasing phonon contribution to octahedral tilting as the temperature increases.

The influence of Fermi level position at the GaN surface on carrier transfer across the MAPbI3/GaN interface.

Both gallium nitride (GaN) and hybrid organic-inorganic perovskites such as methylammonium lead iodide (MAPbI3) have significantly influenced modern optoelectronics. Both marked a new beginning in the development of important branches in the semiconductor industry. For GaN, it is solid-state lighting and high-power electronics, and for MAPbI3, it is photovoltaics. Today, both are widely incorporated as building blocks in solar cells, LEDs and photodetectors. Regarding multilayers, and thus multi-interfacial construction of such devices, an understanding of the physical phenomena governing electronic transport at the interfaces is relevant. In this study, we present the spectroscopic investigation of carrier transfer across the MAPbI3/GaN interface by contactless electroreflectance (CER) for n-type and p-type GaN. The effect of MAPbI3 on the Fermi level position at the GaN surface was determined which allowed us to draw conclusions about the electronic phenomena at the interface. Our results show that MAPbI3 shifts the surface Fermi level deeper inside the GaN bandgap. Regarding different surface Fermi level positions for n-type and p-type GaN, we explain this as carrier transfer from GaN to MAPbI3 for n-type GaN and in the opposite direction for p-type GaN. We extend our outcomes with a demonstration of a broadband and self-powered MAPbI3/GaN photodetector.

Mixology of MA1-x EA x PbI3 Hybrid Perovskites: Phase Transitions, Cation Dynamics, and Photoluminescence.

Mixing molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance the stability and performance of optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed 3D methylammonium (MA)-ethylammonium (EA) MA1-x EA x PbI3 (x < 0.4) hybrid perovskites. We use a suite of different techniques to determine the structural phase diagram, cation dynamics, and photoluminescence properties of these compounds. Upon introduction of EA, we observe a gradual lowering of the phase-transition temperatures, indicating stabilization of the cubic phase. For mixing levels higher than 30%, we obtain a complete suppression of the low-temperature phase transition and formation of a new tetragonal phase with a different symmetry. We use broad-band dielectric spectroscopy to study the dielectric response of the mixed compounds in an extensive frequency range, which allows us to distinguish and characterize three distinct dipolar relaxation processes related to the molecular cation dynamics. We observe that mixing increases the rotation barrier of the MA cations and tunes the dielectric permittivity values. For the highest mixing levels, we observe the signatures of the dipolar glass phase formation. Our findings are supported by density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing, indicating the suitability of these compounds for optoelectronic applications.

Toward h-BN/GaN Schottky Diodes: Spectroscopic Study on the Electronic Phenomena at the Interface.

Hexagonal boron nitride (h-BN), together with other members of the van der Waals crystal family, has been studied for over a decade, both in terms of fundamental and applied research. Up to now, the spectrum of h-BN-based devices has broadened significantly, and systems containing the h-BN/III-V junctions have gained substantial interest as building blocks in, inter alia, light emitters, photodetectors, or transistor structures. Therefore, the understanding of electronic phenomena at the h-BN/III-V interfaces becomes a question of high importance regarding device engineering. In this study, we present the investigation of electronic phenomena at the h-BN/GaN interface by means of contactless electroreflectance (CER) spectroscopy. This nondestructive method enables precise determination of the Fermi level position at the h-BN/GaN interface and the investigation of carrier transport across the interface. CER results showed that h-BN induces an enlargement of the surface barrier height at the GaN surface. Such an effect translates to Fermi level pinning deeper inside the GaN band gap. As an explanation, we propose a mechanism based on electron transfer from GaN surface states to the native acceptor states in h-BN. We reinforced our findings by thorough structural characterization and demonstration of the h-BN/GaN Schottky diode. The surface barriers obtained from CER (0.60 ± 0.09 eV for GaN and 0.91 ± 0.12 eV for h-BN/GaN) and electrical measurements are consistent within the experimental accuracy, proving that CER is an excellent tool for interfacial studies of 2D/III-V hybrids.

Carbon nanotube materials for electrocardiography.

Carbon nanotubes (CNTs) as 1D nanomaterials of excellent physicochemical characteristics bring hope to compete and eventually conquer traditional solutions in electrocardiography - one of the most powerful and non-invasive diagnostic tools in cardiac disorders. Our review tracks (from 2008) the development of CNTs as critical components in the systems where CNTs serve mainly as electroconductive fillers hence enable recording electrocardiographs (ECG). The characteristics of the CNT-based ECG systems - mainly to-skin electrodes and in a few cases wiring - covers their electrical and mechanical performance (adhesivity, flexibility, elasticity) and qualitative biocompatibility. By comprehensive analysis of the state-of-art in this field, we intend to indicate the most important challenges for the CNT (and other) materials to be applied in scale-up solution for electrocardiography in the near future.

Oxidised carbon nanotubes as dual-domain synergetic stabilizers in electroconductive carbon nanotube flexible coatings.

We report that combining oxidised carbon nanotubes (O-CNTs) and pristine CNTs may be the answer for more electroconductive composites. Short (<1 µm) oxidised multi-wall CNTs (O-MWCNTs) acted as an unobvious and excellent conductivity enhancer in MWCNT-based composite thin films. 'Blending' O-MWCNTs (1.5 wt%) with 250 µm-long MWCNTs (98.5 wt%), both of well-defined morphology and physicochemistry, led to a 3- and 26-fold increase in specific conductivity as compared to purely MWCNT- or purely O-MWCNT-based thin films, respectively. We explain the enhanced conductivity by the effect of a dual-domain structure of O-MWCNTs. The scale-up method, i.e. screen-printing, opens a route to application in textronics (i.e. electrical and electronic textiles) and hence targets for medicine, civil/military engineering, wellness, etc.

Covalently immobilized lipase on aminoalkyl-, carboxy- and hydroxy-multi-wall carbon nanotubes in the enantioselective synthesis of Solketal esters.

Aiming at the preparation of efficient, stable on storage and recyclable nanobiocatalysts for enantioselective transesterification, alkaline lipase from Pseudomonas fluorescens was covalently immobilized (up to 8.5wt.%) on functionalized multi-wall carbon nanotubes (f-MWCNTs). f-MWCNTs were synthesized via: (a) (2+1)-cycloaddition of a nitrene to the C-sp(2) nanotube walls (3.2mmolg(-1), a novel synthetic approach) and, (b) oxidative treatments, i.e. Fenton reagent (3.5mmolg(-1)) and nitrating mixture (2.5mmolg(-1)), yielding aminoalkyl-, hydroxyl- and carboxyl-MWCNTs, respectively. Amino- and epoxy- functionalized mesoporous silica (f-SBA-15) were used as the reference supports. Transesterification of vinyl n-butyrate by racemic Solketal with a chromatographically (GC) traced kinetics was selected as the model reaction. The studies revealed that different chemical functionalization of morphologically identical nanotube supports led to various enzyme loadings, catalytic activities and enantioselectivities. MWCNT-NH2-based nanobiocatalyst was found to be the most active composite among all of the tested systems (yield 20%, t=0.5h, 1321Ug(-1)), i.e. 12 times more active than the native enzyme. In turn, lipase immobilized on MWCNT-COOH emerged as the most enantioselective system (ex aequo with SBA-NH2) (eeR=74%, t=0.5h at yield of 3-5%). The activity of the MWCNT-NH2-based nanobiocatalyst after 8 cycles of transesterification dropped to 60% of its initial value, whereas for SBA-NH2-based composite remained unchanged. Importantly, stability on storage was fully maintained for all MWCNT-based nanobiocatalysts or even 'extra-enhanced' for MWCNT-OH.

An electrically controlled drug delivery system based on conducting poly(3,4-ethylenedioxypyrrole) matrix.

As numerous therapeutic agents are not well tolerated when administrated systemically, localized and controlled delivery can help to decrease their toxicity by applying an optimized drug concentration at extended exposure time. Among different types of drug delivery systems, conjugated polymers are considered as promising materials due to their biocompatibility, electrical conductivity and ability to undergo controllable redox reactions. In this work poly(3,4-ethylenedioxypyrrole), PEDOP, matrix is described for the first time as a reservoir of a model drug, ibuprofen (IBU). Drug immobilization process is performed in situ, during the electrochemical polymerization of 10 mM EDOP in the presence of 5-50 mM IBU. The loading efficiency of polymer matrix is dependent on IBU concentration and reaches 25.0±1.3 µg/cm2. The analysis of PEDOP-IBU chemical structure based on Raman spectroscopy, energy dispersive spectroscopy and surface morphology data provided by scanning electron microscopy shows that IBU is accumulated in the structure of matrix and evidently influences its morphology. IBU is then released in a controlled way under the influence of applied potential (-0.7 V vs. Ag/AgCl). It is demonstrated that the judicious choice of the synthesis conditions leads to a tailored loading efficiency of PEDOP matrix and to a tunable drug release.

In Vitro Targeting and Selective Killing of T47D Breast Cancer Cells by Purpurin and 5-Fluorouracil Anchored to Magnetic CNTs: Nitrene-Based Functionalization versus Uptake, Cytotoxicity, and Intracellular Fate.

Fe-encapsulated multiwall carbon nanotubes (Fe@MWCNTs) are candidates for magnetically targeted Drug Delivery Systems (mt-DDSs) against breast cancer. However, their full potential as versatile and biosafe vectors has yet to be developed. Key challenges that remain are relating surface functionalization to cytotoxicity and inducing selective cytotoxicity to cancer cells. We have studied quantitative uptake of pristine and functionalized Fe@MWCNTs (f-Fe@MWCNTs) in correlation to their in vitro cytotoxicity. Human monocyte macrophages (HMMs) and T47D breast cancer cells were selected as models to test selective cytotoxicity. [2+1]-Cycloaddition of nitrenes to Fe@MWCNTs yielded both effective functionalization and drug "tethering". Hydrophilization of Fe@MWCNTs was critical for efficient active cell uptake. f-Fe@MWCNTs were considerably more toxic to T47D cells than HMMs, in spite of longer exposure times of the latter. Eventually, Fe@MWCNTs loaded with 5-fluorouracil in a ß-cyclodextrin cage or with covalently linked purpurin emerged as the most cytotoxic and steerable in a magnetic field toward promising mt-DDSs.

Amalgamation of complex iron(III) ions and iron nanoclusters with MWCNTs as a route to potential T2 MRI contrast agents.

Iron-filled multiwall carbon nanotubes (Fe@MWCNTs) were functionalized toward a variety of potential magnetic resonance imaging contrast agents. Oxidized Fe@MWNCTs were covered with PEG5000 via direct esterification or using acyl chloride derivatives. Alternatively, the latter were functionalized with an aminophenol ligand (Fe@O-MWCNT-L). Moreover, pristine Fe@MWCNTs were functionalized with N-phenylaziridine groups (Fe@f-MWCNT) via [2+1] cycloaddition of nitrene. All of these chemically modified nanotubes served as a vehicle for anchoring Fe(3+) ions. The new hybrids--Fe(III)/Fe@(f-/O-)MWCNTs--containing 6%-14% of the "tethered" Fe(3+) ions were studied in terms of the acceleration of relaxation of water protons in nuclear magnetic resonance. The highest transverse relaxivity r2=63.9±0.9 mL mg(-1) s(-1) was recorded for Fe(III)/Fe@O-MWCNT-L, while for Fe(III)/Fe@f-MWCNT, with r2=57.9±2.9 mL mg(-1) s(-1), the highest impact of the anchored Fe(III) ions was observed. The T1/T2 ratio of 30-100 found for all of the nanotube hybrids presented in this work is a very important factor for their potential application as T2 contrast agents. Increased stability of the hybrids was confirmed by ultraviolet-visible spectrophotometry.