Atomic Model of CdSe QDs Containing Density Waves as Derived from PDF Analysis
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Atomic Model of CdSe QDs Containing Density Waves as Derived from PDF Analysis B. Palosz1, W. Palosz2, P. Wijewarnasuriya3, S. Gierlotka1 , K. Skrobas1, and S. Stelmakh1 1
Institute of High Pressure Physics PAS, ul. Sokolowska 29-37, 01-142 Warsaw, Poland Brimrose Corporation, Sparks, Md 21152, USA 3 Army Research Laboratory, 2800 Powder Mill Road, Adelphi, Md 20783-1197, USA 2
ABSTRACT Using a new methodology of elaboration of PDF data (G(r) function), which is based on the analysis of individual inter-atomic distances (ri), a function describing differences between average inter-atomic distances in CdSe nanograins derived experimentally and those in the parent bulk crystal was determined. Based on that methodology a unique atomic architecture of CdSe QDs is proposed. The results show that a good knowledge about the grain surface of nanocrystals alone may be insufficient for understanding the nanomaterials properties, and that the real atomic structure of the interior of nanograins is of importance as well. INTRODUCTION The uniqueness of nanocrystalline semiconductors is, primarily, in the dependence of their luminescence properties on the grain size, and this feature of quantum dots is extensively explored. Years of experience in research on practical applications of quantum dots show, that a serious impediment in further progress is a limited knowledge about the processes occurring on the nanograin surface during the synthesis and further technological processing. For a better control of the syntheses as well as for a better interpretation of the properties of quantum dots, a good knowledge about the real atomic structure of the materials is of paramount importance. So far attempts to determine structural differences between the nanograin interior and its surface using powder diffraction methods presented in the literature have failed [1-3]. A systematic study of the atomic structure of CdSe nanocrystals performed with application of PDF analysis indicated a presence of internal strains in grains of different sizes [4,5], but neither their origin nor an appropriate atomic model of individual nanograins was offered as yet. EXPERIMENTAL DETAILS Colloidal Quantum Dots (QD) investigated in this work were synthesized using a typical route in non-polar environment. Sample PQD131 was synthesized using Se dissolved in Trioctylphosphine as the Se-precursor, and CdO dissolved in Oleic Acid (Cd oleate) as the Cdprecursor. As the solvent, Octadecene was used. The synthesis took place at 230 °C and lasted 5 minutes. The reaction by-products and excess reagents were removed with ethanol using centrifugation. Samples PQD135 and PQD168 were synthesized similarly as that of PQD131, except that additional ligands, Oleyamine and Tetradecylphosphonic Acid were also used. The syntheses proceeded at 235-245 °C for 4 minutes. Purification was again accomplished by addition of ethanol, then centrifugation, redissolution in hexane, precipitation with ethanol, and second centrifugation, followed by separation of the pellet and its redis
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