Diego E. Gallardo

Light‐Emitting Diodes with Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals are promising luminophores for creating a new generation of electroluminescence devices. Research on semiconductor nanocrystal based light-emitting diodes (LEDs) has made remarkable advances in just one decade: the external quantum efficiency has improved by over two orders of magnitude and highly saturated color emission is now the norm. Although the device efficiencies are still more than an order of magnitude lower than those of the purely organic LEDs there are potential advantages associated with nanocrystal-based devices, such as a spectrally pure emission color, which will certainly merit future research. Further developments of nanocrystal-based LEDs will be improving material stability, understanding and controlling chemical and physical phenomena at the interfaces, and optimizing charge injection and charge transport.

Fabrication and characterization of red-emitting electroluminescent devices based on thiol-stabilized semiconductor nanocrystals

Thiol-capped CdTe nanocrystals were used to fabricate light-emitting diodes, consisting of an emissive nanocrystal multilayer deposited layer by layer, sandwiched between indium tin oxide and aluminum electrodes. The emissive and electrical properties of devices with different numbers of nanocrystal layers were studied. The improved structural homogeneity of the nanocrystal multilayer allowed for stable and repeatable current- and electroluminescence-voltage characteristics. These indicate that both current and electroluminescence are electric-field dependent. Devices were operated under ambient conditions and a clear red light was detected. The best performing device shows a peak external efficiency of 0.51% and was measured at 0.35mA∕cm2 and 3.3V.

In situ antimony doping of solution-grown ZnO nanorods

ZnO nanorods are doped with Sb during the aqueous chemical synthesis by addition of Sb acetate dissolved in ethylene glycol.

Temperature shifted photoluminescence in CdTe nanocrystals

If semiconductor quantum dots are to be incorporated into hetero-structural devices such as light emitting diodes it is important to understand the influences of the surrounding medium on the properties and particularly the photoluminescence of the nanocrystals. Here we investigate the temperature dependence of emission from CdTe quantum dots in aqueous solution with capping layers of thioglycolic acid. Results from quantum dots both held in suspension and deposited as thin films are shown. In both suspensions and thin film multilayers a reversible spectral shift to lower energy is seen with increasing temperature. This red shift of photoluminescence is thought to be the result of increased exciton carrier transfer between the quantum dots at higher temperatures and the thermal activation of emission from lower energy trap states. Both suspension and thin film devices also show a recoverable loss in photoluminescence intensity when the sample is heated. These changes are explained by the thermal activation of nonradiative surface traps. Finally, an irreversible loss in photoluminescence is reported in the CdTe thin film devices and to a lesser extent also in the quantum dot suspensions. This observation is explained by the heat induced formation of agglomerates imaged by AFM analysis.

Light emitting devices based on nanostructured semiconductors

Light emitting devices based on high-efficiency photoluminescence (PL) fluorescent nanocrystals have been investigated in terms of the generation of light from the structure using a variety of deposition methods. An automated modified layer-by-layer (LbL) self-assembly technique has been employed to produce multilayers of thiol-capped red fluorescing CdTe nanocrystals. Indium-tin-oxide (ITO) and aluminium electrodes were used as the electrodes. Morphological characterization was carried out through Schottky field effect (SFEG) SEM and atomic force microscopy (AFM). The structures built presented clear red electroluminescence (EL) to the naked eye. Turn on voltages were found to be in the range of 3-6 volts while the onset current was in the order of tens of microamperes. The role of structure homogeneity, the presence of pinholes and lifetime extension were features addressed during this investigation. Samples with a lifetime of continuous operation in air longer than 60 minutes and highly stable EL spectra were achieved; EL was visible to the unaided eye, although the brightness was still below the commercial standards and has not yet been qualified.

Degradation of Photoluminescence in CdTe Nanoparticles Due to Cl2 Contamination

Nanostructured CdTe is proving a popular material for a variety of modern applications. Here, we report photoluminescent deterioration of CdTe due to Cl2 contamination. Cl2 gas was bubbled through a 2ml suspension of thioglycolic acid capped CdTe nanoparticles whilst photoluminescence was monitored. With the addition of only 0.02ml of Cl2 a loss of luminescence intensity was observed. Addition of 0.4ml of Cl2 resulted in a 70% loss of luminescence intensity, a 21nm shift in wavelength, and a large narrowing of the full-width-at-half-maximum. Cl2 attacks the Cd-S bond in the thiol capping layer of the nanoparticle, disrupting the passivation of the nanocrystal, and allowing the formation of non-radiative trap states. Smaller nanocrystals with a larger surface area to volume ratio are more heavily affected as is shown in the selective reduction of intensity from the smaller particles resulting in a narrower full-width-at-half-maximum and an apparent photoluminescence shift. A decrease in overall intensity is seen due to the lower number of emitting particles.