Sequential Growth of Magic‐Size CdSe Nanocrystals

Abstract

Colloidal semiconductor nanocrystals have been exploited in several applications in which they serve as fluorophores, because of the tunability of the wavelength of the emitted light. [1][2][3] The possibility of exactly controlling the size of nanocrystals is of great importance in the development of these materials, as this will lead to nano-objects with well-defined and reproducible properties. Whereas this goal seems to be hard to achieve with large nanocrystals, it might be viable for clusters consisting of a few tens or hundreds of atoms, as in this size regime a handful of structures can have an exceptionally high stability and therefore would form preferentially over any other combination of atoms. This concept is already well-known for several metal clusters, as for some of them several "magic" structures exist that are formed by closed shells of atoms. [4][5][6][7] Cluster molecules that can be considered as the smallest building units of semiconductors have been investigated in the past.As an example several tetrahedral cluster molecules based on the general formula [z-(where E = S or Se; M = Zn or Cd; and R = alkyl or aryl) or similar were reported some years ago. [8,9] The series was formed only by clusters containing a well-defined number of atoms, and therefore, characterized by particularly stable structures; thus, these structures can also be termed "magic-size clusters" (MSCs). Different families of almost monodisperse CdS clusters of sizes down to 1.3 nm were reported by Vossmeyer et al., [10] whereas CdSe MSCs were observed later in the solution growth of colloidal nanocrystals [11] and the various cluster sizes found were explained as arising from the aggregation of smaller clusters. Soloviev et al. synthesized and crystallized a homologous series of CdSe cluster molecules [12,13] (very similar in structure to those reported earlier [8,9] ) that were capped by selenophenol ligands. Also in many high-temperature organometallic syntheses of colloidal CdSe nanocrystals, either the transient formation of ultrasmall, highly stable CdSe clusters was noticed, [14,15] or these clusters could be isolated using size-selective precipitation. [16,17] Recently, one type of CdSe MSC has been synthesized in a water-in-oil reverse-micelle system.[18]Here, we report a method for controlling the sequential growth in solution of CdSe MSCs of progressively larger sizes. Each of these types of clusters is characterized by a sharp optical-absorption feature at a well-defined energy. During the synthesis, the relative populations of the different families of MSCs varied, as smaller MSCs evolved into larger MSCs. We can model the time evolution of the concentration of the various magic sizes using a modification of a continuous-growth model, by taking into account the much higher stability of the various MSCs over nanocrystals of any intermediate size.For the synthesis of the CdSe MSCs reported here a mixture of dodecylamine and nonanoic acid was used to decompose cadmium oxide at 200°C under an inert atmosphere. Th...

Homogeneously-Alloyed CdTeSe Single-Sized Nanocrystals with Bandgap Photoluminescence

“…Note that the absorption and emission maxima of the MSQD ensemble are independent of the growth periods during the reaction (as shown in the Supporting Information, Figure S1), implying stability in size with a constant bandgap; the growth pattern shows the typical characteristics of MSQDs. [13][14][15][16][17][18][19][20][21][22][23] Thus, the value of the energy difference of the present absorption doublet of the CdTeSe MSQDs is larger than those of the CdSe MSQDs and smaller than that of the CdTe MSQDs. Such a difference in the value of the energy difference of the absorption doublet suggests that the present MSQDs represent a ternary system.…”

“…4 In fact, they were discovered as molecular cluster-like precursors leading to the formation of larger aggregates with various morphologies such as wire-like. [13][14][15][16] It is noteworthy that, for those II-VI MSQDs reported, [17][18][19][20][21][22] they exhibited no bandgap but trap emission; and very often, several families of different sizes coexisted in one synthetic batch. 19,20 It was not until recently that PbSe MSQDs were claimed to have bandgap emission, but unfortunately with broad bandwidth, 23 which might result from the nature of the broad homogeneous fluorescence line width.…”

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

“…In this context, ultrasmall nanocrystals have the ability to (1) serve as active precursors in the formation of various anisotropic nanostructures, 15,[19][20][21] and (2) undergo fast charge separation and slow recombination. [22][23][24] Despite immense interest in ultrasmall nanocrystals in fundamental chemical processes, only a handful of high temperature [25][26][27] or alkylphosphine [28][29] precursor-based synthetic methods have been developed to prepare them. Very recently, low temperature, nonphosphinated methods for synthesis of ultrasmall CdSe nanocrystals were reported.…”

Density Functional Study on the Morphology and Photoabsorption of CdSe Nanoclusters

“…The present work has been focused of the (CdSe) 33 cluster which is the smallest CdSe nanocrystal isolated and characterized by mass spectrometry 16,17 . For this cluster, the UV-Vis absorption spectrum is experimentally available (Figure 10a) and it has been employed as a benchmark to test the model systems 2,16,18 .…”