RESUMO
Solid-solid interfacial processes greatly affect the performance of electronic and composite materials, but probing the dynamics of buried interfaces is challenging and often involves lengthy or invasive sample preparation. We show that bilayer nanoribbons-made here of tin dioxide and copper-are convenient structures for observing as-made interfaces as they respond to changing temperature in a transmission electron microscope (TEM). At low temperatures (<200 degrees C), differential thermal expansion causes the bilayers to bend when heated or cooled, with the motion determined by the extent of Cu-SnO(2) epitaxy. At higher temperatures, we are able to watch-in real time and with nanometer resolution-a progression of grain growth, interdiffusion, island formation, solid-state chemical reactions, and melting. This novel TEM geometry is readily applicable to other nanoribbon/coating combinations and is well suited to observing interfacial phenomena driven thermally or by the application of mechanical, electrical, or magnetic forces.
Assuntos
Cobre/química , Temperatura Alta , Nanopartículas Metálicas/química , Nanotecnologia/instrumentação , Nanotecnologia/métodos , Ligas/química , Carbono/química , Cristalização , Microanálise por Sonda Eletrônica , Teste de Materiais , Microscopia Eletrônica de Transmissão , Óxidos/química , Temperatura , Estanho/química , Compostos de Estanho/químicaRESUMO
We describe the structural, optical, and electrical properties of high-quality films of PbSe nanocrystals fabricated by a layer-by-layer (LbL) dip-coating method that utilizes 1,2-ethanedithiol (EDT) as an insolubilizing agent. Comparative characterization of nanocrystal films made by spin-coating and by the LbL process shows that EDT quantitatively displaces oleic acid on the PbSe surface, causing a large volume loss that electronically couples the nanocrystals while severely degrading their positional and crystallographic order of the films. Field-effect transistors based on EDT-treated films are moderately conductive and ambipolar in the dark, becoming p-type and 30-60 times more conductive under 300 mW cm(-2) broadband illumination. The nanocrystal films oxidize rapidly in air to yield, after short air exposures, highly conductive p-type solids. The LbL process described here is a general strategy for producing uniform, conductive nanocrystal films for applications in optoelectronics and solar energy conversion.
Assuntos
Cristalização/métodos , Eletroquímica/métodos , Chumbo/química , Membranas Artificiais , Mercaptoetanol/análogos & derivados , Nanoestruturas/química , Nanotecnologia/métodos , Fotoquímica/métodos , Compostos de Selênio/química , Condutividade Elétrica , Luz , Substâncias Macromoleculares/química , Teste de Materiais , Mercaptoetanol/química , Conformação Molecular , Nanoestruturas/ultraestrutura , Tamanho da Partícula , Refratometria , Espalhamento de Radiação , Propriedades de SuperfícieRESUMO
We study multiple exciton generation (MEG) in electronically coupled films of PbSe quantum dots (QDs) employing ultrafast time-resolved transient absorption spectroscopy. We demonstrate that the MEG efficiency in PbSe does not decrease when the QDs are treated with hydrazine, which has been shown to greatly enhance carrier transport in PbSe QD films by decreasing the interdot distance. The quantum yield is measured and compared to previously reported values for electronically isolated QDs suspended in organic solvents at approximately 4 and 4.5 times the effective band gap. A slightly modified analysis is applied to extract the MEG efficiency and the absorption cross section of each sample at the pump wavelength. We compare the absorption cross sections of our samples to that of bulk PbSe. We find that both the biexciton lifetime and the absorption cross section increase in films relative to isolated QDs in solution.