Mechanistic Study of the Synthesis of CdSe Nanocrystals: Release of Selenium

We outline a reaction pathway for the cleavage of the PSe bond in trialkylphosphine selenide during the synthesis of CdSe nanocrystals. The reaction between cadmium carboxylate and trimethylphosphine selenide in the presence of an alcohol produces alkoxytrimethylphosphonium (<b>2</b>). Control experiments and density functional theory calculations suggested that the cleavage of the PSe bond is initiated by nucleophilic attack of carboxylate on a Cd²⁺-activated phosphine selenide to produce an acyloxytrialkylphosphonium intermediate (<b>1</b>), which is converted to <b>2</b> in the presence of an alcohol

A Nuclear Magnetic Resonance Study of the Binding of Trimethylphosphine Selenide to Cadmium Oleate

We report an NMR study on the binding of trimethylphosphine selenide (SePMe₃) to cadmium oleate (Cd­(OA)₂) in CDCl₃ and toluene-<i>d</i>₈. At room temperature in CDCl₃, SePMe₃ binds to Cd­(OA) ₂ in 1:1 ratio with a binding constant of 20 ± 3 as determined by NMR titration. The Cd-bound and free SePMe₃ are in fast exchange on the NMR time scale at room temperature and gives only one ³¹P NMR peak. At ca. 190 K, three ³¹P NMR peaks were observed for a toluene-<i>d</i>₈ solution of 1:1 mixture of Cd­(OA)₂ and SePMe₃. These three peaks were tentatively assigned to free SePMe₃ (9.0 ppm), 1:1 (19.5 ppm), and 2:1 complex between SePMe₃ and Cd­(OA)₂ (18.8 ppm)

Mechanistic Insights into the Role of Alkylamine in the Synthesis of CdSe Nanocrystals

This paper reports a detailed mechanistic study of the effect of alkylamine on the synthesis of CdSe nanocrystals. Alkylamines are one of the most important additives for the synthesis of colloidal semiconductor nanocrystals. However, their effect on the monomer production as well as nanocrystal nucleation and growth are not well understood, as indicted by inconsistent and contradictory conclusions in the literature. We found that alkylamines slow down the reaction between cadmium oleate and trialkyl phosphine selenide by binding to cadmium and preventing the activation of trialkyl phosphine selenide. A linear correlation was observed between the observed reaction rate constant and the ³¹P NMR chemical shift or ¹<i>J</i>_P–Se of phosphine selenide. In the presence of alkylamine, an alkylaminophosphonium intermediate was observed. Mechanistic study suggests that the cleavage of PSe bond is through nucleophilic attack by carboxylate instead of alkylamine. Interestingly, although alkylamines decrease the rate of monomer production, it increases the rate of CdSe nanocrystal growth. Although seemingly contradictory, this is due to a drastic decrease in the nanocrystal nucleation events in the presence of alkylamines. As a result, each nucleus is fed with more monomers and grows faster in the presence of alkylamine than in its absence

Ten Most Frequent Insight Words.

<p>Ten Most Frequent Insight Words.</p

How Does Word Length Evolve in Written Chinese?

<div><p>We demonstrate a substantial evidence that the word length can be an essential lexical structural feature for word evolution in written Chinese. The data used in this study are diachronic Chinese short narrative texts with a time span of over 2000-years. We show that the increase of word length is an essential regularity in word evolution. On the one hand, word frequency is found to depend on word length, and their relation is in line with the Power law function y = ax^-b. On the other hand, our deeper analyses show that the increase of word length results in the simplification in characters for balance in written Chinese. Moreover, the correspondence between written and spoken Chinese is discussed. We conclude that the disyllabic trend may account for the increase of word length, and its impacts can be explained in "the principle of least effort".</p></div

How does language change as a lexical network? An investigation based on written Chinese word co-occurrence networks

<div><p>Language is a complex adaptive system, but how does it change? For investigating this process, four diachronic Chinese word co-occurrence networks have been built based on texts that were written during the last 2,000 years. By comparing the network indicators that are associated with the hierarchical features in language networks, we learn that the hierarchy of Chinese lexical networks has indeed evolved over time at three different levels. The connections of words at the micro level are continually weakening; the number of words in the meso-level communities has increased significantly; and the network is expanding at the macro level. This means that more and more words tend to be connected to medium-central words and form different communities. Meanwhile, fewer high-central words link these communities into a highly efficient small-world network. Understanding this process may be crucial for understanding the increasing structural complexity of the language system.</p></div

The diachronic change of the MUL of Chinese (measured based on words) and the MWL of Chinese (measured based on characters).

<p>The diachronic change of the MUL of Chinese (measured based on words) and the MWL of Chinese (measured based on characters).</p

The Means of Causal, Insight Words and Quantifiers.

<p>The Means of Causal, Insight Words and Quantifiers.</p

Dynamic and static mean word length evolution for different text scales.

<p>Dynamic and static mean word length evolution for different text scales.</p

Evolution of the clustering coefficient (<i></i>) and the average path length (<i></i>).

<p>Evolution of the clustering coefficient (<i></i>) and the average path length (<i></i>).</p