Z. T. Liu

Different origins of visible luminescence in ZnO nanostructures fabricated by the chemical and evaporation methods

We prepared ZnO nanostructures using chemical and thermal evaporation methods. The properties of the fabricated nanostructures were studied using scanning electron microscopy, x-ray diffraction, photoluminescence, and electron paramagnetic resonance (EPR) spectroscopy. It was found that the luminescence in the visible region has different peak positions in samples prepared by chemical and evaporation methods. The samples fabricated by evaporation exhibited green luminescence due to surface centers, while the samples fabricated by chemical methods exhibited yellow luminescence which was not affected by the surface modification. No relationship was found between green emission and g∼1.96 EPR signal, while the sample with yellow emission exhibited strong EPR signal.

Change of the emission spectra in organic light-emitting diodes by layer thickness modification

Electroluminescence and photoluminescence of organic light-emitting diodes consisting of an indium tin oxide anode, N,N8-di(naphthalene-1-yl)-N,N8-diphenyl-benzidine as a hole transport layer, tris (8-hydroxyquinoline) aluminum as emitting layer, and an Ag cathode were measured for different layer thickness values. It was found that, for a certain range of thickness values, multiple peak emission can be achieved. In addition, the emission spectra were dependent on the viewing angle. For the optimized thickness values, normal incidence chromaticity coordinates achieved were 0.32 and 0.43. Possible explanations for observed unexpected behavior are discussed.

Response to “Comment on ‘Change of the emission spectra in organic light-emitting diodes by layer thickness modification’” [Appl. Phys. Lett. 86, 186101 (2005)]

In our recent paper, 1 we presented the study of the emission spectra of triss8-hydroxyquinolined aluminum sAlqd based organic light-emitting diodes sOLEDsd as a function of organic layer thickness. Both calculations and experimental results were presented. The discrepancy between the calculated and measured emission spectra was noted, and possible reasons were discussed. Finally, it was concluded that further study is needed to conclusively establish whether any other phenomena in addition to simple interference play a role in the obtained results. In his recent comment, 2 Shore claimed that our experimental data can be entirely explained by simple interference phenomena, and presented calculations which qualitatively “reproduce” basic behavior of our devices. However, the calculations in our work fFig. 1sbdg also show similar behavior as those presented in the comment by Shore sFig. 2d. Since Shore 2 changed only the Alq layer thickness, while we considered devices with different N, N8-disnaphthalene-1-yl d-N,N8-diphenylbenzidinesNPBd and Alq thicknesses, direct comparison can only be made for the device with 65 nm NPB and 139 nm Alq. It is obvious from Fig. 1sbd in our letter and Fig. 2 in the recent comment 2 that both calculations show essentially the same features. Yet, quantitative agreement between the experimental data and the calculated results is lacking. Since the thicknesses of organic layers were verified by step profiler and spectroscopic ellipsometry after deposition, thickness errors in the fabricated devices are unlikely. Another possible reason for the discrepancy, as correctly identified by Shore, 2 is different emission region thickness. Figure 1 illustrates the influence of the assumed emission region thickness on the calculated electroluminescence sELd spectra. Further studies with confined emitting layers of known thickness are in progress to experimentally establish the influence of the emitting layer thickness. We would also like to point out that, regardless of the assumed emission layer thickness, the calculated spectra always show two peaks, while some of the experimental spectra showed clear shoulders in addition to two peaks. Comparison between the calculated and the experimental spectra for the 65/153s65 nm NPB and 153 nm Alq d device is shown in Fig. 2sad. It can be observed that the calculated spectrum, exhibiting a two-peak structure, does not describe the experimental spectrum well. The calculation for the same

Angular dependence of the emission from low Q-factor organic microcavity light emitting diodes

Microcavity light emitting diodes based on materials with broad emission spectrum, such as tris (8-hydroxyquinoline) aluminum (Alq3), typically exhibit a significant blue shift of the emission wavelength with increasing viewing angle. In this work, we investigate the influence of the organic layer thickness on the angular dependence of the emission spectrum in low Q-factor microcavity OLEDs. We demonstrate that for different organic layer thicknesses qualitatively different emission wavelength dependences on the viewing angle can be obtained in microcavity devices consisting of two organic layers, N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transport layer and Alq3 as emitting layer. The devices with different organic layer thickness were characterized by electroluminescence, photoluminescence, reflectance, and transmission measurements and results compared with the model.

Green photoluminescence in ZnO nanostructures

In photoluminescence (PL) spectrum of ZnO, typically one or more peaks in the visible spectral range due to defect emission can be observed in addition to one UV peak due to band edge emission. The origin of the defect emission is controversial and several mechanisms have been proposed. In this work, we fabricated ZnO nanostructures with different methods (evaporation and chemical synthesis). We found that the preparation method influences the peak position of the defect emission. Different hypotheses for the origin of the green emission in our nanostructured samples are discussed.