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 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.