Effect of Ag/Al co-doping method on optically p-type ZnO nanowires synthesized by hot-walled pulsed laser deposition

Silver and aluminum-co-doped zinc oxide (SAZO) nanowires (NWs) of 1, 3, and 5 at.% were grown on sapphire substrates. Low-temperature photoluminescence (PL) was studied experimentally to investigate the p-type behavior observed by the exciton bound to a neutral acceptor (A⁰X). The A⁰X was not observed in the 1 at.% SAZO NWs by low-temperature PL because 1 at.% SAZO NWs do not have a Ag-O chemical bonding as confirmed by XPS measurement. The activation energies (E_a) of the A⁰X were calculated to be about 18.14 and 19.77 meV for 3 and 5 at.% SAZO NWs, respectively, which are lower than the activation energy of single Ag-doped NW which is about 25 meV. These results indicate that Ag/Al co-doping method is a good candidate to make optically p-type ZnO NWs.     

Improved efficiency and lifetime for green phosphorescent organic light-emitting diodes using charge control layer

We investigated green phosphorescent organic light-emitting diodes (PHOLEDs) with charge control layer (CCL) to produce high efficiency and improve operational lifetime. Three types of devices were fabricated following the number of CCL within emitting layer (EML), maintaining the thickness of whole EML. The CCL and host material, which was 4,4′-bis (carbazol-9-yl)biphenyl (CBP) with bipolar property, can control carrier movement in EML. Therefore, the electroluminescent (EL) performance improvement as efficiency and lifetime was realized with a good charge balance, an effective triplet exciton confinement, and the reduced triplet exciton quenching effect in EML. Device 2 with a CCL as exciton distribution structure exhibits the remarkable EL performances for the maximum luminous and external quantum efficiency of 65.34 cd/A and 20.42%, respectively. Moreover, operational lifetime is nearly improved 2.5 times than the conventional device.     

Electron emission from CVD diamond p-i-n junctions with negative electron affinity during room temperature operation

We successfully observed electron emission from hydrogenated diamond p-i-n junction diodes with negative electron affinity during room temperature operation. The emissions started when the applied bias voltage produced flat-band conditions, where the capacitance-voltage characteristics showed carrier injection in the i-layer. In this low current injection region, the electron emission efficiency (η) of the p-i-n junction diodes (p is top layer) was about 5 × 10^− 5, while that of the n-i-p diodes (n is top layer) was about 10^− 8. With increasing diode current, both diodes showed an increase in η and a nonlinear increase in emission current. In the high current injection region with high diode current of 5-50 mA, both diodes had an emission current of almost 10 μA, where η of a p-i-n junction diode was 0.18%, while that of a n-i-p junction diode was 0.02%.Note that η, which corresponds to the electron emission mechanism, depended on the diode current level.     

High-quality homoepitaxial diamond (100) films grown under high-rate growth condition

We present advantages of high-power microwave plasma chemical vapor deposition (MPCVD) in homoepitaxial diamond film deposition. Diamond films grown at comparatively high growth rate of 3.5 μm/h showed intense free-exciton recombination emission at room temperature. The free-exciton decay time of the diamond film at room temperature, 22 ns, was much longer than that of type-IIa single crystal, indicating electronically high quality of the homoepitaxial films. Dislocation-related emissions were locally observed, a part of which created by mechanical polishing process was successfully removed by surface etching process using oxygen plasma. Another advantage of the high-power MPCVD is effective impurity doping; boron-doped diamond films with high carrier mobility and high carrier concentration were reproducibly deposited. An ultraviolet photodetector fabricated using the high-quality undoped diamond film showed lower noise equivalent power as well as higher photoresponsivity for ultraviolet light with better visible-blind property, compared to those of standard Si-based photodetectors. The high-power MPCVD is, thus, indispensable technique for depositing high quality diamond films for electronic devices.     

Charge-carrier relaxation dynamics of poly(3-hexylthiophene)-coated gold hybrid nanoparticles

Poly(3-hexylthiophene)-coated gold (Au@P3HT) hybrid nanoparticles have been prepared via a “grafting-to” approach, and compared with pristine P3HT to investigate the dynamics of exciton relaxation using time-resolved transient-absorption and fluorescence spectroscopy. In efforts to facilitate the efficient dissociation of photo-generated excitons, we have incorporated gold nanoparticles having surface-plasmon resonances into thiol-terminated P3HT to fabricate Au@P3HT nanocomposites. The first-excited singlet state of Au@P3HT decays faster than that of pristine P3HT due to interactions with surface-plasmon resonances; excitons undergo dissociation via energy transfer from P3HT to the surface-plasmon state of a gold nanoparticle in Au@P3HT nanocomposites. The lowest triplet state of P3HT is also less populated due to the energy transfer while its lifetime is slightly reduced by the presence of gold nanoparticles. Thus, it is suggested that our incorporation of gold nanoparticles into P3HT reduces the recombination of geminate excitons and, thereby, increases the probability of exciton dissociation.     

Time-resolved near-field optics: exciton transport in semiconductor nanostructures

We present a systematic, temperature-dependent study of excitonic real-space transfer into single GaAs quantum wires using time-resolved low-temperature near-field luminescence spectroscopy. Excitons generated by local short pulse optical excitation in a 250 nm spot undergo diffusive transport over a length of several micrometres and are subsequently trapped into the quantum wire by optical phonon emission. The effect of local energy barriers in the vicinity of the quantum wire on the real-space transfer dynamics is monitored directly by mapping the time-resolved quantum wire luminescence. Experiments at variable temperatures are compared to numerical simulations based on drift-diffusive model calculations, and the spatio-temporal evolution of the two-dimensional exciton distribution within the nanostructure is visualized.     

Research progress on g–C3N4–based photocatalysts for organic pollutants degradation in wastewater: From exciton and carrier perspectives

Graphitic carbon nitride (g-C₃N₄) can be used to degrade organic pollutants in wastewater owing to its excellent chemical stability, tunable band structure, and visible light (VL) responsiveness. Nevertheless, the application of pristine g-C₃N₄ is significantly hindered by its low specific surface area, unsatisfactory VL utilization (<460 nm), high exciton binding energy (E_b ≈ 0.2 eV), and low photoinduced carrier separation. Development of strategies to overcome these limitations and improve the degradation efficiency has attracted considerable attention. Considering the principles of photoexcitation, this paper describes two unique photoexcitation processes, the carrier and exciton processes, of which the strong exciton effect of g-C₃N₄ has not been extensively discussed in prior reviews. In addition, we comparatively present the latest progress and related mechanisms of the modification strategies such as morphology controlling, elemental doping, defect engineering, and heterojunction building employed in polluted water treatment are compared from the carrier and exciton perspectives. Finally, the unique characteristics, challenges, and future directions are discussed. This review presents research progress on the photoexcitation processes involved in g–C₃N₄–based materials, and proposes that the modification based on the exciton effect and direct verification of the transfer mechanism in the heterojunction can be potential areas of future investigations.