Algirdas Lazauskas

A single emitting layer white OLED based on exciplex interface emission

A new triaryl molecule based on a benzene–benzothiadiazole–benzene core has been applied in a WOLED device. This very simple molecule emits from a combination of emissive states (exciton/electromer/exciplex/electroplex) to give white light with CIE coordinates of (0.38, 0.45) and a colour temperature of 4500 K.

Nine-ring angular fused biscarbazoloanthracene displaying a solid state based excimer emission suitable for OLED application

A new biscarbazoloanthracene consisting of nine fused aromatic rings, including two pyrrole units, has been obtained in a straightforward and convergent synthesis. Computational chemistry and conformational analysis revealed that the semiconductors molecule is not planar, the two carbazole moieties being helical twisted from the plane of the anthracene unit. Photophysical and electrochemical measurements showed that this angular fused heteroacene has a low lying HOMO energy level with a wide band gap despite its extended π-conjugated molecular framework. Based on its relatively low-lying HOMO level, the semiconductor promises a high environmental stability in comparison to other related linear fused acenes and heteroacenes. The biscarbazoloanthracene has been applied as the light emitting layer in a white light emitting diode (WOLED). It is proposed that the white OLED feature is due to dual light emission properties from the active semiconductor layer being based on both the molecular luminescence of the small molecule and a discrete excimer emission made possible by suitable aggregates in the solid state. Noteworthy, this is the first reported example of such a behavior observed in a small molecule heteroacene rather than an oligomer or a polymer.

Green and red phosphorescent organic light-emitting diodes with ambipolar hosts based on phenothiazine and carbazole moieties: photoelectrical properties, morphology and efficiency

The two low-molar mass compounds 10,10′-(9-ethyl-9H-carbazole-3,6-diyl)bis(10H-phenothiazine) and 10-(9-ethyl-9H-carbazol-3-yl)-10H-phenothiazine were synthesized as ambipolar hosts for phosphorescent organic light emitting diodes. The structure and properties of these compounds were studied by X-ray crystallographic analysis, extensive UV-vis and fluorescence spectrometry, cyclic voltammetry and theoretical calculations. These compounds exhibited triplet energies of 2.6 eV; the shallow ionization potentials in the interval of 5.10–5.25 eV. The ambipolar semiconductor properties of the amorphous layers for both compounds were proved by the experimental and Marcus hooping theory methods. Importantly, the compounds appeared to be ideally suited for the green and red phosphorescent organic light-emitting diodes as the hosts for Ir(III) dyes. The maximum power and external quantum efficiencies up to 47.5/40.6 lm W−1 and 20.0/10.5% were observed for the green and red devices, respectively. The quenching processes such as a triplet–triplet annihilation and triplet-polaron quenching contributing to the roll-off of the quantum efficiency were investigated. The considerable widths of the exciton formation zone of 13 and 17 nm were estimated for the best performance devices. The morphology of the physical vapor deposited layers of the different composition mixtures of the hosts and Ir(III) emitters was examined using atomic force microscopy. Based on the obtained morphological information the preliminary correlation between the changes in a surface morphology and the widths of the exciton formation zone were established.

Bactericidal effect of graphene oxide/Cu/Ag nanoderivatives against Escherichia coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Staphylococcus aureus and Methicillin-resistant Staphylococcus aureus.

A systematic analysis of antibacterial activity of individual nanoderivatives, e.g. GO nanosheets, Ag and Cu nanoparticles (NPs), as well as combinations of Cu-Ag NPs, and GO-Cu-Ag nanocomposites against Escherichia coli, Pseudomonas aeruginosa, Staphylococcus aureus, Klebsiella pneumoniae and Methicillin-resistant Staphylococcus aureus (MRSA) was performed. Chemical properties of the GO, Cu and Ag NPs were determined employing X-ray photoelectron spectroscopy and X-Ray-excited Auger electron spectroscopy. Morphology of corresponding nanoderivatives was studied employing transmission electron microscopy and scanning electron microscopy. It was shown that combination of Cu and Ag NPs, as well as GO-Cu-Ag nanocomposite material possess enhanced antibacterial activity through a possible synergy between multiple toxicity mechanisms. MRSA showed highest resistance in all cases.

New WOLEDs based on π-extended azatrioxa[8]circulenes

New stable WOLEDs based on π-extended azatrioxa[8]circulenes have been fabricated. Combining the own blue emission of the azatrioxa[8]circulenes with the yellow-green emission of the “m-MTDATA:azatrioxa[8]circulene” exciplex a broad visible region, from 400 to 700 nm, is covered. The so constructed WOLEDs exhibit a luminance exceeding 23 700 cd m−2 and an external quantum efficiency reaching 3%.

Multicolor Luminescence Switching and Controllable Thermally Activated Delayed Fluorescence Turn on/Turn off in Carbazole–Quinoxaline–Carbazole Triads

We report a series of thermally activated delayed fluorescence (TADF) molecules with mechanochromic luminescence properties and reversible TADF turn on/off properties in solid state that are induced by the transition between amorphous and crystalline states. Additionally, multicolor altering through external stimulus is demonstrated. All of the studied compounds exhibited recovery of the initial states associated with narrower emission spectra. TADF organic light-emitting diodes fabricated by solution processing rendered high external quantum efficiency up to 10.9% and luminance of 16 760 cd m-2.

Nitrogen-doped twisted graphene grown on copper by atmospheric pressure CVD from a decane precursor

We present Raman studies of graphene films grown on copper foil by atmospheric pressure CVD with n-decane as a precursor, a mixture of nitrogen and hydrogen as the carrier gas, under different hydrogen flow rates. A novel approach for the processing of the Raman spectroscopy data was employed. It was found that in particular cases, the various parameters of the Raman spectra can be assigned to fractions of the films with different thicknesses. In particular, such quantities as the full width at half maximum of the 2D peak and the position of the 2D graphene band were successfully applied for the elaborated approach. Both the G- and 2D-band positions of single layer fractions were blue-shifted, which could be associated with the nitrogen doping of studied films. The XPS study revealed the characteristics of incorporated nitrogen, which was found to have a binding energy around 402 eV. Moreover, based on the statistical analysis of spectral parameters and the observation of a G-resonance, the twisted nature of the double-layer fraction of graphene grown with a lower hydrogen feeding rate was demonstrated. The impact of the varied hydrogen flow rate on the structural properties of graphene and the nitrogen concentration is also discussed.

UV-Curable Aliphatic Silicone Acrylate Organic–Inorganic Hybrid Coatings with Antibacterial Activity

The most effective means to protect against bacterial invasion and to reduce the risk of healthcare-associated infections are antibacterial components synthesis. In this study, a novel process for the synthesis of organic–inorganic hybrid coatings containing silver nanoparticles is presented. Silver nanoparticles and polymer formation proceeds simultaneously through the in situ photoreduction of silver salt to silver nanoparticles and UV-crosslinking of bifunctional aliphatic silicone acrylate. The nanocomposite films with 0.5–1.43 wt % of silver nanoparticles concentration were obtained and investigated. The formation of silver nanoparticles in polymer matrix was confirmed via UV-visible spectroscopy, X-ray diffraction (XRD), Fourier transform infrared spectroscopy, scanning electron spectroscopy, and energy dispersive spectroscopy. Our investigations clearly show the formation of silver nanoparticles in silicone acrylate network. Direct photoreduction of silver salt by UV-radiation in the organic media produced silver nanoparticles exhibiting cubic crystal structure. The size of nanoparticles was determined to be near 20 ± 5 nm. The antibacterial activities of coatings were determined using the disc diffusion and direct contact methods. UV-curable silicone acrylate hybrid coatings exhibited antibacterial activity against harmful bacteria strains.

Synthesis and Characterization of Silver-Poly(Methylmethacrylate) Nanocomposite

Silver nanoparticles and polymethylmethacrylate (PMMA) nanocomposite was prepared in situ by photo-induced thermal reduction method. The interfacial interaction of Ag nanoparticles and PMMA polymer is investigated using Fourier transform infrared spectroscopy (FTIR). Optical properties of Ag/PMMA films were characterized by UV-Vis and FTIR absorption spectroscopy. Effects of the UV and heat-treatment time on the formation of silver nanoparticles in PMMA matric matrix were studied in detail. These investigations proposed new nanocomposite structures. They can be defined as plasmonic materials with improved optical properties. Ag/PMMA structures may found a number of technological applications: in optical devices, various plasmonic sensors or even in nanomedicine.

Advanced design of UV waveplates based on nano-structured thin films

Optical elements for polarization control are one of the main parts in advanced laser systems. The state and intensity of polarized light is typically controlled by optical elements, namely waveplates. Polymers, solid or liquid crystals and other materials with anisotropic refractive index can be used for production of waveplates. Unfortunately, most of aforementioned materials are fragile, unstable when environmental conditions changes, difficult to apply in microsystems and has low resistance to laser radiation. Retarders, fabricated by evaporation process, do not consist any of these drawbacks. In order to manufacture such optical components with high quality, characterisation of deposition parameters are essential. A serial bi-deposition method was employed to coat anisotropic layers for polarisation control. Such waveplate can be deposited on micro optics or other optical elements, essentially improving compact optical systems. The range of available materials is limited by absorption losses for waveplates in UV spectral region. Therefore, the investigation was accomplished with four eligible candidates – TiO2, LaF3, Al2O3 and SiO2. Structural (XPS, XRD) and optical (spectrophotometry, ellipsometry) analysis have shown Al2O3 and SiO2 as the most applicable materials for UV spectral region.