Semiconductor nanocrystal stability

Recently in the field of physics of semiconductors and materials science a great attention has been paid to formation and optical properties of semiconductor nanocrystals (quantum dots, QDs) dispersed in inorganic matrixes. An interest to glassy materials with QDs is associated with their unique physical properties and possibility to create elements of optoelectronic devices. Phase separation processes followed by crystallization are the basic in production of such materials. They result in formation of semiconductor nanocrystals stabilized within a glass matrix. The materials are advanced for various applications because of optical and thermal stability and possibility to control optical features through the technology of glass preparation and post-synthesis thermal treatment. 

The reactions used for synthesis of II-VI (CdSe, CdTe), III-V (InP, InAs), and IV-VI (PbS, PbSe, PbTe) semiconductor nanocrystals are outlined by Schemes 3-5.4,17,30-32 The syntheses are carried out at high temperatures, and in the presence of long-chain alkylphosphines (trioctylphosphine,TOP), alkyl-phosphine oxides (trioctylphosphine oxide, TOPO), alkylamines (hexadecyl-amine, HD A), and alkylphosphonic acids as the stabilizing agents. 

Abstract We have achieved selective gas sensing based on different size semiconductor nanocrystals incorporated into rationally selected polymer matrices. From the high-throughput screening experiments, we have found that when CdSe nanocrystals of different size (2.8 and 5.6nm diameter) were incorporated into different types of polymer fdms, the photoluminescence (PL) response patterns upon laser excitation at 407-nm and exposure to polar and nonpolar solvent vapors were dependent on the nature of polymer. We analyzed the spectral PL response from both sizes of CdSe nanocrystals using multivariate analysis tools. Results of this multivariate analysis demonstrate that a single film with different size CdSe nanocrystals serves as a selective sensor. The stability of PL response to vapors was evaluated upon 16h of continuous exposure to laser excitation. 

The research in nanoelectronic materials is driven by the need to tailor electronic and optical properties for specific components in nanotechnology. In this respect, semiconductor nanocrystals (NCs) surfacely passivated by organic molecules are candidates for possible practical applications. A success in usage of NC-organic composites depend on the understanding their optical and photophysical characteristics as well as their surface/interface properties and stability [1]. 

Fluorescent semiconductor nanocrystals (CdSe, CdTe, PbSe, and others), otherwise included in the term quantum dots (QDs), have attracted much attention in various research fields for more than 20 yeais owing to their chemical and physical properties, which differ markedly fi om those of the bulk solid (quantum size effect). Quantum dots have size-tuneable light emission (usually with a narrow emission band), bright luminescence (high quantum yield), long stability (photobleaching resistance), and broad absorption spectra for simultaneous excitation of multiple fluorescence colors compared with classical organic fluorescent dyes. 

It is well known that the capping layer which stabilizes the QDs may also interact with the analyte, thus affecting the luminescence properties of the semiconductor nanocrystals. Subsequently, oleic acid-coated GlSe QDs were synthesized and dispersed in chloroform, and the nitroaromatic explosives 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), nitrobenzene (NB), 2,4-dinitrochlorobenzene were each intercalated between the hydrophobic layer surrounding the QDs. The quenching of the luminescence of the nanocrystals by the intercalated nitro-substituted analytes, via an ET mechanism, enabled a simultaneous analysis of the different explosives [133]. 

The chemical and physical properties of thiol-stabilized II-VI semiconductor nanocrystals were reviewed by EychmuUer and Rogach [30]. The materials prepared... 

A dielectric oxide layer such as silica is useful as shell material because of the stability it lends to the core and its optical transparency. The thickness and porosity of the shell are readily controlled. A dense shell also permits encapsulation of toxic luminescent semiconductor nanoparticles. The classic methods of Stober and Her for solution deposition of silica are adaptable for coating of nanocrystals with silica shells [864,865]. These methods rely on the pH and the concentration of the solution to control the rate of deposition. The natural affinity of silica to oxidic layers has been exploited to obtain silica coating on a family of iron oxide nanoparticles including hematite and magnetite [866-870]. The procedures are mostly adaptations of the Stober process. Oxide particles such as boehmite can also be coated with silica [871]. Such a deposition process is not readily extendable to grow shell layers on metals. The most successful method for silica encapsulation of metal nanoparticles is that due to Mulvaney and coworkers [872—875]. In this method, the smface of the nanoparticles is functionalized with aminopropyltrimethylsilane, a bifunctional molecule with a pendant silane group which is available for condensation of silica. The next step involves the slow deposition of silica in water followed by the fast deposition of silica in ethanol. Changes in the optical properties of metal nanoparticles with silica shells of different thicknesses were studied systematically [873 75]. This procedure was also extended to coat CdS and other luminescent semiconductor nanocrystals [542,876-879]. 

Recently, DHBCs have been used as a good stabilizer for the in-situ formation of various metal nanocolloids and semiconductor nanocrystals such as Pd, Pt [328-330], Au [280,328-330], Ag [331], CdS [332], and lanthanum hydroxide [333]. PAA-fe-PAM and PAA-fc-PHEA were used as stabihzer for the formation of hairy needle-Uke colloidal lanthanum hydroxide through the complexation of lanthanum ions in water and subsequent micelhzation and reaction [333]. The polyacrylate blocks induced the formation of starshaped micelles stabilized by the PAM or PHEA blocks. The size of the sterically stabilized colloids was controlled by simply adjusting the polymer-to-metal ratio, a very easy and versatile synthesis strategy for stable colloids in aqueous environment [333]. The concept of induced micelhzation of anionic DHBCs by cations was also apphed in a systematic study of the direct synthesis of highly stable metal hydrous oxide colloids of AP+, La +, Ni +, Zn ", Ca ", or Cu " via hydrolysis and inorganic polycondensation in the micelle core [334,335]. The AP+ colloids were characterized in detail by TEM [336], and the intermediate species in the hydrolysis process by SANS, DLS, and cryo-TEM [337]. 

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