On the basis of evidence from 31P NMR spectroscopy, and using PbSe as a model, we propose two simultaneous mechanisms through which "monomers" are formed in preparations of lead chalcogenide nanocrystals (NCs). In one mechanism, selenium is delivered as a Se2- species, whereas in the other, Se0 reacts with metal already reduced by the organophosphine. This latter mechanism helps explain the sensitivity of NC preparations to the purity of organophosphines and allows the rational modification of batch NC reactions to increase yield.

Alloy particles 1 can exhibit electronic, 2-4 optical, 5-10 and catalytic properties 11,12 that are distinct from those of the corresponding mono-metal particles, 13-17 prompting numerous preparations of multi-metal nanoparticles, including those that can be considered core/shell bimetallic, partially segregated alloy, and pure alloy. 1-12 These earlier alloy particles typically required specialized equipment or handling procedures, posed difficulties in isolation and analysis, and could not be redissolved in airstable forms.This paper describes a simple synthesis of nanometer-sized monolayer-protected alloy clusters (MPACs) that are the first examples of stable, large, alloy molecules that can be isolated in solvent-free forms and redissolved without change. The stable alkanethiolate monolayer is the key to preventing metal core aggregation. The MPAC core compositions can be systematically varied in regards to ratios and numbers of groups 10 (Pt, Pd) and 11 (Cu, Ag, Au) metals, creating a pathway to studying the S0002-7863(98)01454-1 CCC: $15.00

Colloidal semiconductor nanocrystals (NCs) have been extensively studied in recent years for use in a variety of applications including biological fluorescent labels, [1] electroluminescent devices, [2] and lasers. [3] For any of these applications, it is essential to begin with high-quality NCs, and advances in the synthesis of II±VI and III±V NCs have made it possible to prepare relatively monodisperse, highly crystalline samples. In preparations with organometallic precursors, NCs are often prepared in a batch process in which the precursors are rapidly injected into a heated flask containing a mixture of solvents and coordinating ligands. [4±6] However, the quality and average size of NCs synthesized in the batch process can depend strongly on factors which are difficult to control such as the injection process, local temperature and concentration fluctuations, rate of stirring, and rate of cooling. In a continuous-flow system, reactions are performed at steady state, making it possible to achieve better control and reproducibility. Further benefits can be realized by scaling down the reactor dimensions to micrometers, thereby reducing the consumption of reagents during the optimization process and improving the uniformity of temperature and residence times within the reaction volume. A microfluidic flow reactor is attractive for NC synthesis because it is possible to rapidly and continuously screen through important reaction parameters, while using minimal amounts of reagents, until NCs of the desired size and monodispersity are produced. In contrast, each set of parameters would represent a separate reaction if the optimization procedure were conducted using a batch process. The inherent advantages of a microfluidic flow system also make it a natural choice for extracting kinetic data on NC nucleation and growth processes, information which has been difficult to obtain using conventional, macroscale batch methods. In spite of the perceived advantages of flow systems, it has been difficult to simply adopt the chemistry used in batch preparations of semiconductor NCs to a microfluidic flow reactor. There have been a few reports on the preparation of II± VI NCs in flow systems, [7±9] but these reports have not demonstrated the wide optical tunability, low polydispersities, and high quantum yields attainable in the batch process, nor have they extracted new kinetic data on particle formation. CdSe is probably the most well characterized colloidal semiconductor NC system because its effective bandgap can be tuned over the majority of the visible region. However, existing preparations are generally not amenable to a continuous flow system. In the most widely used preparation of high quality CdSe quantum dots, dimethylcadmium and tri-n-octylphosphine selenide (TOPSe) are rapidly injected into a hot solvent consisting of a mixture of tri-n-octylphospine (TOP) and tri-n-octylphosphine oxide (TOPO).[4] The solvent also serves as the source of surface ligands for the growing NCs. This method ensures that nucleati...

Microfluidic reactors enable a number of advantages over conventional chemical processes including enhanced control of heat and mass transfer, lower reagent consumption during optimization, and sensor integration for in-situ reaction monitoring. [1,2] Reactors are usually fabricated from either silicon, glass, or polymers; those made of silicon or glass are advantageous because they can tolerate a broad range of chemistries and high temperatures. Microreactors for the large class of homogeneous liquid-phase reactions are often based on single-phase laminar flow designs in which reagent streams are brought into contact. However, such designs are limited in terms of slow diffusive reagent mixing and broad residence time distributions (RTDs). Recirculation within segments in a two-phase segmented flow approach (gasliquid or liquid-liquid) overcomes such limitations by providing a mechanism of exchanging fluid elements located near the channel walls with those at the center. [3][4][5] This recirculatory motion has the dual effect of narrowing the RTD and improving reactant mixing. In contrast to single-phase designs, segmentation makes it possible to drive reactions to required yields over significantly shorter times owing to the enhanced mixing, while maintaining narrow RTDs.