Eric A. Meulenkamp

Identification of the transition responsible for the visible emission in ZnO using quantum size effects

The emission properties of suspensions of nanocrystalline ZnO particles with different particle sizes were studied. Two emission bands were observed, one being an exciton emission and the other the visible emission of ZnO. The energy of both emissions depends on the particle dimensions due to size quantization. A linear relationship between the energetic maxima of the two emission bands is found. Because of the difference in effective masses of electrons and holes in ZnO, the slope of the linear relationship clearly indicates that the visible emission is due to the transition of an electron from the conduction band to a deep trap. The nature of the deep trap is also considered.

The luminescence of nanocrystalline ZnO particles: the mechanism of the ultraviolet and visible emission

Results of steady-state luminescence measurements performed on suspensions of nanocrystalline ZnO particles of different sizes are presented. In all cases two emission bands are observed. One is an exciton emission band in the UV and the second an intense and broad emission band in the visible, shifted by approximately 1.5 eV with respect to the absorption onset. As the size of the particles increases, the intensity of the visible emission decreases, while that of the exciton emission increases. In accordance with previous results, a model is presented in which the visible emission is assigned to the radiative recombination of an electron from a level close to the conduction band edge and a deeply trapped hole in the bulk (V .. o ) of the ZnO particle. The size dependence of the intensity ratio of the visible to exciton luminescence and the kinetics are explained by a model in which the photogenerated hole is transferred from the valence luminescence and the kinetics are explained by a model in which the photogenerated hole is transferred from the valence band to a V . o level in the bulk of the particle in a two-step process. The first step of this process is an efficient surface-trapping, probably at on O 2- site.

In-situ X-ray diffraction of Li intercalation in sol–gel V2O5 films

Abstract We have performed an electrochemical and in-situ X-ray diffraction study of Li intercalation in crystalline V 2 O 5 thin films to find out if these films, prepared by the sol–gel route using vanadium oxo-isopropoxide as precursor, show different Li x V 2 O 5 phases than V 2 O 5 prepared by other techniques. For x =0.0 and x =0.4 the results are identical. The phases found can be described as α-V 2 O 5 ( x = 0.0) and e-Li x V 2 O 5 ( x = 0.4). However, for a larger degree of intercalation some noticeable differences became apparent. First, the phase for x =0.8 showed an elongated c -axis compared to e-Li x V 2 O 5 ( x =0.8). Moreover, it is probably monoclinic. Second, the electrochemical and structural data do not provide evidence for the formation of δ-LiV 2 O 5 ( x =1.0). Third, the phase for x =1.4 bears some resemblance to δ-LiV 2 O 5 . The reason for these differences is sought in the structure and chemistry of the present V 2 O 5 films. It is suggested that this is subtly different from that of ‘standard’ crystalline material, owing to the low-temperature sol–gel preparation route.

Electrical and optical properties of TiO2 in accumulation and of lithium titanate Li0.5TiO2

Changes in the optical absorption and electrical conductivity of dense and mesoporous anatase TiO2 films were measured in situ as a function of electrode potential during electrochemical lithium intercalation. A special two-electrode geometry was used for the conductivity measurements, in which the contacts were separated by a small gap bridged by the TiO2. When electrons are injected, an accumulation layer is formed and the conductivity increases several orders of magnitude. A monotonic increase of the optical absorption with wavelength confirms the presence of (partially) delocalized electrons. Insertion of lithium ions results in the formation of the Li0.5TiO2 phase and a decrease of the overall conductance. The specific conductivity of the Li0.5TiO2 phase is (9.1±0.2) S/cm, significantly lower than that of Li-doped anatase TiO2. This is corroborated by the absorption spectrum of Li0.5TiO2, which shows two pronounced peaks around 440 and 725 nm and no characteristic free-electron features. At potential...

The Electrochromic Behavior of Indium Tin Oxide in Propylene Carbonate Solutions

The authors report on a study of transparent conducting tin-doped indium oxide (ITO) electrodes in propylene carbonate solutions containing lithium ions. The system was studied using electrochemical methods in combination with in situ techniques: ultraviolet-visible spectroscopy, X-ray diffraction, and quartz crystal microbalance. The results show that the cathodic process at E {approx_gt} 1.0 V vs. Li/Li{sup +} mainly involves the reduction of the electrolyte solution, leading to the formation of a thin, lithium-rich surface film. At potentials {approx_lt}1.0 V vs. Li/Li{sup +}, degradation of ITO and the formation of metallic indium take place. No evidence was obtained that lithium-ion intercalation into ITO, which has been suggested by several workers, occurs to a significant extent. The authors conclude that ITO probably cannot be used as a combined ion-storage layer and transparent conductor for all-solid-state and laminated electrochromic switching devices in view of long-term stability.

21.1: Ink Jet Printing of Passive‐Matrix Polymer Light Emitting Displays

The requirement and characteristics for red, green and blue (RGB) electroluminescent polymers[1,2] suitable for fabricating monochrome and full-color passive-matrix polymer display[3] will be discussed. The controlled deposition of the light-emitting polymers is accomplished through the use of high-resolution ink jet printing[4,5], where the materials are first tested in devices prepared using spincoating techniques. The specifications for polymer materials, as well as the display design which must be taken into account to make a high-resolution passive-matrix display with low power consumption are presented.

High efficiency polymer LEDs: triplets and novel devices

We present results and a discussion of highly efficient polymer Light-Emitting Diodes (polymer LEDs, PLEDs). The external quantum efficiency in current standard devices reaches up to 2-4% only. We have explored two routes to enhance this value. In the first route, PEDOT/PSS is replaced with a novel anode or hole injection layer. The efficiency with some Light Emitting Polymers (LEP) is improved significantly, resulting in an efficacy of 35 cd/A for a yellow emitting poly-(para-phenylene-vinylene) and 20 cd/A for a blue emitting poly-(spirobifluorene). We attribute the major improvement compared to standard devices, where about 10 and 5 cd/A are obtained, respectively, to a combination of improved exciton formation efficiency and light out-coupling efficiency, and to less quenching of the radiative decay under actual device operating conditions. In the second route, we developed a new host polymer with high triplet energy such that transition metal-based green-emitting phosphorescent dyes can be used without significant back transfer of triplet excitons to the polymer host. First results using this system showed about 25 cd/A using a soluble green Ir-based emitter. Importantly, all data are obtained in a standard two-layer device of a hole transport/injection layer and the LEP.

Electrochemically Induced Characteristic Luminescence of Metal Ions at Anodic Valve Metal Oxides

Metal ions, such as Hg 2+ , Pb 2+ , Mn 2+ , Eu 3+ , and Tb 3+ , can show emission at anodic valve metal oxides in the presence of an oxidizing agent. The luminescence is characteristic of the ions and agrees well with the luminescence in crystalline solids. Various observations indicate that both the oxide and the electrolyte species play a decisive role in the occurrence of emission. Hence, the emission must take place at the oxide/solution interface. A tentative mechanism is proposed involving reaction steps well known in electroluminescence and electrogenerated chemiluminescence

Technology and materials for full-color polymer light-emitting displays

The status of the development of full-color polymer light emitting diodes will be presented. The focus of current materials research is on state-of-the-art red, green, and blue light-emitting polymers (LEP) with high efficiency, optimum color points, low driving voltages and long lifetimes in devices. A general overview of the progress of red, green and blue LEP lifetimes and efficiencies will be given and compared to requirements for both full-color passive and active matrix-displays for mobile display applications. Further, the status of ink-jet printing of LEPs for the industrialization of full-color displays will be discussed, and a comparison of the performance of spin coated and inkjet printed devices will be presented. In addition, two material-related topics studied recently will be discussed; namely, the lifetime of blue light-emitting devices correlated to processing, anodes, cathodes and the blue polymers themselves, and second, the consequences of pulsed-driving schemes on efficiency and lifetime.

Passive-matrix polymer light-emitting displays

Organic light-emitting device research focuses on the use of small-molecule and polymer materials to make organic electroluminescent displays, with both passive- and active-matrix technologies. This paper will focus on the characteristics of red, green, and blue electroluminescent polymers suitable for fabricating monochrome and full-color passive-matrix displays. The stability of polymer OLEDs, and the use of ink-jet printing for direct high-resolution patterning of the light-emitting polymers will also be discussed. It will be shown that the performance of light-emitting polymers is at the brink of being acceptable for practical applications.