Facet-selective growth and optical properties of CdTe/CdSe tetrapod-shaped nanocrystal heterostructures
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Vladimir V. Roddatis and Alexander L. Vasiliev
Russian Research Centre “Kurchatov Institute,” Moscow 123182, Russia
Alexei G. Vitukhnovsky Department of Luminescence, Lebedev Physical Institute, Moscow 119991, Russia
Alexander M. Gaskov Department of Chemistry, Moscow State University, Leninskie Gory, Moscow 119991, Russia (Received 6 April 2011; accepted 3 June 2011)
Selective CdSe tip growth on CdTe tetrapod-shaped colloidal seeds has been achieved for a Cd: surfactant molar ratio of 1:2, where surfactant is oleic acid. The average length of tetrapod arms increased from 12 to 21 nm while arm width remained constant of 3 nm. Formation of CdSe tips shifts the excitonic absorption maximum to the near-infrared region and the appearance of low-intensity absorption feature corresponding to a charge-transfer band. At the same time, luminescence band splits into a narrow (about 100 meV width) CdTe excitonic subband and a 230-meV-wide charge-transfer subband, with splitting energy increasing up to 260 meV depending on CdSe tip length. The intensity ratio of charge transfer to excitonic luminescence increases exponentially with splitting energy rise. Considerable modification of the photoluminescence spectrum has been observed with temperature variation in the range of 10–60 °C.
I. INTRODUCTION
Size-dependent electronic and optical properties of colloidal semiconductor nanocrystals (colloidal quantum dots) have been intensively studied over the last decade. A number of outstanding studies on various semiconductor nanocrystals from simple spherical nanocrystals1 to objects with more complicated shape2–4 and composition5–9 have been reported. Modern colloidal chemistry approaches allow synthesizing nanocrystals with various architectures such as nanorods,4 nanowires,10 octapods,11 nanodumbbells,12 tetrapods,13 and hyperbranched nanocrystals.14 Shape control was realized because of crystal structure polymorphism and selective stabilizer attachment.2 The ability to tune nanocrystal shape in a predictable manner and to control the optical properties by shape variation is of significant fundamental and practical interest. In our study, we focused on CdTe/CdSe colloidal nanocrystals with epitaxial heterojunction, so-called colloidal nanocrystal heterostructures. Spherical CdTe/CdSe core/shell nanocrystals show near-infrared emission with energy lower than that of semiconductor components and long luminescence lifetime because of electron-hole spatial separation.5 Conduction and valence band align-
ment of CdTe/CdSe heterostructure result in preferred localization of the photoexcited electron in the CdSe part, whereas hole is mostly confined to the CdTe part. The degree of carriers separation increases as CdSe shell thickness became larger. In nanocrystal heterostructures, emission wave length and extinction coefficients may be tuned independently by varying core size and shell thickness.15,16 Such objects can find practical applications as biolabels, laser media, and materials for photovoltaics. On the other hand, for anisotrop
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