Size/Shape-Controlled Synthesis of Colloidal CdSe Quantum Disks: Ligand and Temperature Effects

Abstract

Size/shape-controlled colloidal CdSe quantum disks with zinc-blende (cubic) crystal structure were synthesized using air-stable and generic starting materials. The colloidal CdSe quantum disks were approximately square, and their lateral dimensions were varied between 20 and 100 nm with the thickness controlled between 1 and 3 nm, which resulted in sharp and blue-shifted UV-vis and PL peaks due to one-dimensional quantum confinement. The quantum disks were grown with either <001> or <111> direction, polar directions in the single crystalline disks, as the short axis, and both basal planes were terminated with Cd ions. These surface Cd ions were passivated with negatively charged fatty acid ligands to neutralize the net positive charges caused by the excess monolayer of Cd ions. The coordination of the Cd ions and carboxylate groups further enabled the close-packing monolayer of fatty acid ligands on each basal plane. The close packing of the hydrocarbon chains of fatty acids dictated the up temperature limit for synthesis of the colloidal quantum disks, and the low temperature limit was found to be related to the reactivity of the starting materials. Overall, a high Cd to Se precursor ratio, negative-charged fatty acid ligands with a long hydrocarbon chain, and a proper temperature range (approximately between 140 and 250 °C) were found to be needed for successful synthesis of the colloidal CdSe quantum disks.

Mechanistic Study of the Formation of Bright White Light-Emitting Ultrasmall CdSe Nanocrystals: Role of Phosphine Free Selenium Precursors

“…It is known that synthesis of QDs in the presence of anionic X-type ligands, e.g., the benzoate ion, results in surface metal cation rich nanocrystals to maintain the overall charge neutrality of ligand-coated QDs. [44][45][46][47][48][49] We believe that the existence of a similar atomic structure at the surface of our ultrasmall nanocrystals influenced the peak sharpness for 1S(e)-2S3/2(h) transition, as was recently demonstrated for CdSe QDs. 44 The ~8 nm red shift in the broad emission band of our nanocrystals also indicates that their surface was composed of different ligands, which influenced exciton delocalization as will be discussed later.…”

Colloidal Synthesis of Ultrathin Two‐Dimensional Semiconductor Nanocrystals

“…[ 12 ] The quantum disks were synthesized by the reaction between cadmium fatty acid salt and elemental selenium, which is similar to that of CdSe nanoplatelets reported by the Dubertret group. However, the acetate salt, which is the key reagent for the formation of 2D CdSe nanoplatelets, was not needed to obtain the CdSe quantum disks.…”

“…More interestingly, the optical properties of these 2D NCs were very similar to those of CdSe quantum belts reported by the Buhro group, [ 9 ] CdSe nanoplatelets reported by the Dubertret group, [ 10 , 11 ] and CdSe quantum disks reported by the Peng group. [ 12 ] All the 2D CdSe NCs reported so far exhibit very sharp and intense fi rst and second excitonic transitions ( Figure 3 A). The Dubertret group and the Buhro group suggested that the two peaks in [ 11 ] Copyright 2011, American Chemical Society.…”

“…[ 9 ] On the other hand, 2D CdSe NCs with a cubic zinc-blende structure were reported by the Dubertret group and the Peng group. [10][11][12] Interestingly, all of these ultrathin 2D CdSe NCs [6][7][8][9][10][11][12] exhibit very similar optical properties. Here, we review recent advances in the synthesis, formation mechanism, and optical properties of ultrathin 2D cadmium chalcogenide (mainly CdSe) NCs.…”

Comparison of the observed size-dependent melting point of CdSe nanocrystals to theoretical predictions

“…In our study, the CdSe nanocrystal surface was passivated with oleic acid, which bonds to the surface via a carboxylic acid. Previous synthetic work on CdSe nanocrystals has demonstrated that the choice of surface ligands can influence the overall shape of a nanocrystal due to the surface ligands binding more tightly at certain facets [16,26]. It is possible that ligands which bind more tightly to the surface would prevent the surface atoms from rearranging at elevated temperatures; this could lead to nanocrystals of the same size having different melting points based upon their surface passivation.…”