Tomas Serevičius

Optical Characterization of MBE-Grown ZnO Epilayers

Optical characterization of molecular-beam-epitaxy-grown ZnO and MgZnO epitaxial layers on sapphire (0001) substrates is presented. The parameters such as carrier recombination time and optical gain coefficient are analyzed. Radiative recombination mechanisms of ZnO in dense quasiparticle system are discussed. The ZnO epilayers even with lower structural quality are tolerable for applications in optoelectronics as light emitters.

Inactivation of bacterial biofilms using visible-light-activated unmodified ZnO nanorods

Various zinc oxide (ZnO) nanostructures are widely used for photocatalytic antibacterial applications. Since ZnO possesses a wide bandgap, it is believed that only UV light may efficiently assist bacterial inactivation, and diverse crystal lattice modifications should be applied in order to narrow the bandgap for efficient visible-light absorption. In this work we show that even unmodified ZnO nanorods grown by an aqueous chemical growth technique are found to possess intrinsic defects that can be activated by visible light (λ = 405 nm) and successfully applied for total inactivation of various highly resistant bacterial biofilms rather than more sensitive planktonic bacteria. Time-resolved fluorescence analysis has revealed that visible-light excitation creates long-lived charge carriers (τ > 1 μs), which might be crucial for destructive biochemical reactions achieving significant bacterial biofilm inactivation. ZnO nanorods covered with bacterial biofilms of Enterococcus faecalis MSCL 302 after illumination by visible light (λ = 405 nm) were inactivated by 2 log, and Listeria monocytogenes ATCL3C 7644 and Escherichia coli O157:H7 biofilms by 4 log. Heterogenic waste-water microbial biofilms, consisting of a mixed population of mesophilic bacteria after illumination with visible light were also completely destroyed.

Room temperature phosphorescence vs thermally activated delayed fluorescence in carbazole – pyrimidine cored compounds

Utilization of “dark” triplet states is a key task for design of novel emissive organic compounds. Thermally activated delayed fluorescence (TADF) and room temperature phosphorescence (RTP) are two promising pathways to employ triplet excitons and reach high internal quantum efficiencies. In this paper we present a series of carbazole–pyrimidine cored compounds, showing RTP and TADF. A specific energy level scheme was shown to promote intersystem crossing and RTP in carbazole–pyrimidine derivatives with phosphorescence quantum yield values of up to 0.07 in rigid Zeonex films. The modification of the acceptor core with 2-methylthio and dimethylamine fragments allowed us to reduce the singlet–triplet energy gap, enhance the rISC (reverse intersystem crossing) rate and obtain high intensity TADF with a fluorescence quantum yield of up to 0.32. Time resolved fluorescence analysis revealed the presence of conformational disorder of the donor–acceptor core governing TADF properties in solid films. A method to control the conformational disorder by enhancing the host rigidity was demonstrated. Sky-blue electroluminescence was shown for the dimethylamino-modified compound with the most intense TADF. Our results show that carbazole–pyrimidine cored materials are versatile donor–acceptor systems for achieving RTP and TADF via simple structural modifications.