Quantum-Dot Films Display Strong Photoluminescence
Daniel S. Burgess
A sol-gel fabrication technique developed at the National Institute of Advanced Industrial Science and Technology in Ikeda, Japan, yields thin films of quantum dots that exhibit photoluminescent efficiencies comparable to that of semiconductor nanocrystals in an aqueous colloidal solution. The approach may have applications in the production of LEDs for displays and lighting and of photovoltaic cells.
In the self-assembly process, the interaction of a thioglycolic acid (TGA) capping layer on the 3- to 4-nm-diameter quantum dots with 3-aminopropyltrimethoxysilane (APS) or 3-mercaptopropyltrimethoxysilane (MPS) directs the growth of the layers and determines the packing density of the dots. Samples are alternately dipped for five to 10 minutes in a toluene solution of APS or MPS and in an aqueous colloidal solution of TGA-capped CdTe or ZnSe particles. Each dipping cycle adds a 5-nm-thick layer of immobilized, closely packed dots.
To demonstrate the method, the researchers grew films of red-, green- and blue-emitting quantum dots on glass slides and on the inside and outside surfaces of glass bottles. Under 365-nm excitation, the films emitted light centered at 648, 549 and 402 nm, with full width half maxima that were unchanged from the colloidal solutions in the case of the green and blue dots and that were lower in the case of the red. The response in all instances — most so in the case of the red films — was slightly redshifted from that of the colloidal solutions, which the team attributed to energy transfer between the packed dots.
Although the measured photoluminescence efficiencies of the films were approximately half those of the colloidal solutions for all colors, correcting for the difference in refractive index between the solid film and water revealed that the efficiencies of the dots were roughly the same in both cases.
Norio Murase, a senior researcher at the institute, suggested that the method is particularly promising as a replacement for rare-earth phosphors in solid-state lighting applications, given the ease of coating encapsulants suggested by the fabrication of the films on the bottles in the experiments. Moreover, quantum dots have a much shorter emission decay lifetime than the phosphors that convert UV radiation in today’s LEDs, so they are more resistant to brightness saturation and thus could tolerate higher excitation powers, yielding higher output powers.
Langmuir, online Aug. 19, 2005, doi:10.1021/la050397q.
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