THE CONTRIBUTION OF PARTICLE CORE AND SURFACE TO STRAIN, DISORDER ANDVIBRATIONS IN THIOLCAPPED CDTE NANOCRYSTALS

Thiolcapped CdTe nanocrystals (18 Angstrom diameter) are investigated by extended x-ray absorption fine structure (EXAFS) measurements betwe en 8 and 290 K at the Cd and Te K-edge. The different coordination sit es of Cd in the particle core and at the surface are identified and ar e consistent with a CdTe particle core which is covered by a 6d-SR sur face layer (R=organic rest), We are able to study individually changes of the properties of the interior and the surface of the nanoparticle with respect to bulk material, Structure and dynamics of the CdTe nan ocrystals are mainly altered by the requirement of heteroepitaxial gro wth at the interface between the CdTe core and the Cd-SR shell. As a c onsequence, bond lengths and Debye temperatures of particle core and s urface show a tendency to accommodate in thiolcapped CdTe nanocrystals . The trends in bond lengths variation observed in the experiment can be reproduced by calculations of the strain distribution induced by th e lattice mismatch in a simplified isotropic model of a spherical CdTe nanoparticle which is encapsulated by a CdS bulklike shell. The exper iment also shows a significantly enhanced static disorder both in part icle core and surface. In contrast, the asymmetry of the radial pair d istribution function of the Cd-S surface bonds is strongly elevated wh ereas the Cd-Te bonds in the interior of the particle show no enhancem ent with respect to bulk material. Experimental coordination numbers a t the Cd and Te K-edge and the Cd/Te edge jump ratio are in good agree ment with the expected values of a Cd54Te32(SR)(52)(8-)-nanocrystal. T his structural model is a larger homolog of a series of analogous CdS nanoparticles and consists of a CdTe tetrahedron which is partially co ated by a Cd-SR surface layer. (C) 1998 American Institute of Physics.