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Nano Lett ; 8(9): 2954-8, 2008 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-18690751


The ability to tailor the properties of semiconductor nanocrystals through creating core/shell heterostructures is the cornerstone for their diverse application in nanotechnology. The band-offsets between the heterostructure components are determining parameters for their optoelectronic properties, dictating for example the degree of charge-carrier separation and localization. So far, however, no method was reported for direct measurement of these factors in colloidal nanocrystals and only indirect information could be derived from optical measurements. Here we demonstrate that scanning tunneling spectroscopy along with theoretical modeling can be used to determine band-offsets in such nanostructures. Applying this approach to CdSe/CdS quantum-dot/nanorod core/shell nanocrystals portrays its type I band structure where both the hole and electron ground state are localized in the CdSe core, in contrast to previous reports which predicted electron delocalization. The generality of the approach is further demonstrated in ZnSe/CdS nanocrystals where their type II band alignment, leading to electron-hole separation, is manifested.

Nano Lett ; 8(2): 678-84, 2008 Feb.
Artigo em Inglês | MEDLINE | ID: mdl-18179278


We demonstrate tuning of the electronic level positions with respect to the vacuum level in colloidal InAs nanocrystals using surface ligand exchange. Electrochemical as well as scanning tunneling spectroscopy measurements reveal that the tuning is largely dependent on the nanocrystal size and the surface linking group, while the polarity of the ligand molecules has a lesser effect. The implications of affecting the electronic system of nanocrystal through its capping are illustrated through prototype devices.

Cristalização/métodos , Nanoestruturas/química , Nanoestruturas/ultraestrutura , Nanotecnologia/instrumentação , Pontos Quânticos , Transferência de Energia , Desenho de Equipamento , Análise de Falha de Equipamento , Teste de Materiais , Nanotecnologia/métodos , Tamanho da Partícula , Propriedades de Superfície
Nanotechnology ; 19(6): 065201, 2008 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-21730694


We studied the electronic level structure of assemblies of InAs quantum dots and CdSe nanorods cross-linked by 1,4-phenylenediamine molecules using scanning tunneling spectroscopy. We found that the bandgap in these arrays is reduced with respect to the corresponding ligand-capped nanocrystal arrays. In addition, a pronounced sub-gap spectral structure commonly appeared which can be attributed to unpassivated nanocrystal surface states or associated with linker-molecule-related levels. The exchange of the ligands by the linker molecules also affected the structural array properties. Most significantly, clusters of close-packed standing CdSe nanorods were formed.

Nano Lett ; 6(10): 2201-5, 2006 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-17034083


The electronic level structure of colloidal InAs quantum dots (QDs) in two-dimensional arrays, forming a QD-solid system, was probed using scanning tunneling spectroscopy. The band gap is found to reduce compared to that of the corresponding isolated QDs. Typically, the electron (conduction-band) ground state red shifts more than the hole (valence-band) ground state. This is assigned to the much smaller effective mass of the electrons, resulting in stronger electron delocalization and larger coupling between electron states of neighboring QDs compared to the holes. This is corroborated by comparing these results with those for InAs and CdSe nanorod assemblies, manifesting the effects of the electron effective mass and arrangement of nearest neighbors on the band gap reduction. In addition, in InAs QD arrays, the levels are broadened, and in some cases their discrete level structure was nearly washed out completely and the tunneling spectra exhibited a signature of two-dimensional density of states.

Arsenicais/química , Cristalização/métodos , Índio/química , Modelos Químicos , Pontos Quânticos , Semicondutores , Arsenicais/análise , Simulação por Computador , Condutividade Elétrica , Transporte de Elétrons , Índio/análise , Teste de Materiais