Thiols stability

A similar route has been followed most recently by Weiss et al. who have fabricated a multi-island SE device from self-assembled ID gold nanocrystal chains [28]. They assembled thiol-stabilized 50 nm Au particles in a chainlike structure and used subsequently electron-beam lithography for electrode fabrication. Here it has to be... 

Figure 4. Preparative procedure of alkene thiol-stabilized gold nanoparticles. 50% of hexenethiol bounded to the particle surface is hydrogenated to hexanethiol [11b].

Rogach, A.L., Katsikas, L., Kornowski, A., Su, D., Eychmuller, A., and Weller, H. (1996) Synthesis and characterization of thiol-stabilized CdTe nanocrystals. Ber. Bunsenges. Phys. Chem. 100, 1772-1778. 

Three general preparative schemes are of particular interest due to their success in preparing nanoparticles on the order of 1 to 3 nm. The first, commonly known as the Brust method for preparing thiol stabilized Au nanoparticles, is discussed in detail in the chapter by Zhong et al. in Section IV of this book. The second method, which originates from Prof El-Sayed s group, is noteworthy for preparing particles with extremely well-defined shapes (tetrahedra, cubes, etc). ° The third method. 

This section is divided into two main subsections. The first describes the synthesis and characterization of phosphine-stabilized gold clusters. The second focuses on the synthesis and characterization of thiol-stabilized gold clusters and the study of other stabilizers such as arsines or boranes are described. 

A major concern for the use of these quantum dots in liquid crystal applications is their stability. Weller et al. using NMR spectroscopy, UV-vis spectrophotometry, and analytical ultracentrifugation (AUC) showed that thiol-stabilized cadmium chalcogenide quantum dots are unstable at low particle concentrations (<10 pM in DMF), i.e., even the covalently bound thiols desorb from the quantum dot surface... 

During the formation of nanoparticles, surfactants interact with the surfaces of the particles in a dynamic equilibrium process. On the contrary, ligands that are chemisorbed to the surface of the particle are less prone to desorption compared to physisorbed ones. A first consequence of a low desorption rate is that particles do not grow rapidly after nucleation. As an example, when thiols are chemisorbed to AuNPs particle sizes are typically limited to <5 nm. In contrast, the majority of surfactants used to stabilize nanoparticles associate with the surface of particles through van der Waals or other weak interactions, allowing the preparation of larger particles. The first report of thiol-stabilized AuNPs appeared in 1993 by Giersig and Mulvaney.145... 

McCormick [5] prepared thiol-stabilized nanoparticles containing gold, platinum, palladium, rhodium, ruthenium, osmium, and iridium, which were used in optics, immunodiagnostics, and electronics. 

The processes of nucleation and growth in aqueous systems are conceptually similar to those observed in their higher-temperature counterparts. Typically, the metal perchlorate salt is dissolved in water, and the thiol stabilizer is added (commonly, 1-thioglycerol). After the pH is adjusted to > 11 (or from 5-6 if ligand is a mercaptoamine ) and the solution is deaerated, the chalcogenide is added as the hydrogen... 

Small molecules such as phosphines and alkane thiols stabilize metal nanoparticles in a very effective maimer. Very stable covalent metal-phosphorus or metal sulfur bonds lead to such strong ligand shells that in some cases the protected particles can even be isolated in solid state, which can never be done with electrostatically stabilized particles. The chemical nature of the protecting ligand molecules is responsible for the solubility of the particles. Thus, the use of organic solvents has become very useful for several reasons. Figme 3 shows a sketch of the three types of steric stabihzations of metal nanoparticles. 

So-called superlattices of 5mn alkylthiolate protected silver particles having truncated octahedral shapes as well as thiol-stabilized 5 6 mn gold particles can be obtained from solution. Fractional crystallization is a very usual method to separate chemical compounds from other compounds and impurities. Mixtures of thiolate stabilized gold nanoparticles between 1.5 and 3.5 nm could successfully be fractioned into real monodisperse species containing 140, 225, 314, and 459 atoms. 2D assemblies have also become available of these fcc-structured nanoparticles. The decisive criterion to successhilly fraction and crystallize metal nanoparticles is to protect them perfectly by strongly bound ligand molecules in order to avoid coalescence. 

Numerous examples of ordered 2D structures, especially of thiol-stabilized silver and gold nanoparticles, have become known in the course of the last few years. They usually form spontaneously on Transmission Electron Microscopy (TEM) grids or other surfaces from solution and will not be... 

Absorption spectra of PEPC and PVC thin films with nanoparticles are characterized by unstructured long-wave shoulder that comes with the involving of nanoparticles (Fig. 1). The use of the thiol-stabilized particles leads to slight blue shift of the maximum in the range of 400-550 nm for polymers. These effects are more apparent in PEPC films than in PVC ones. 

The IR characterization of the films provides useful information for estimation of the materials structure. Addition of silver and gold nanoparticles to polymers shifted the band to the 1500-1600 cm range, that is characteristic for the stretching vibrations of rr-system of benzene rings. In the case of thiol-stabilized nanoparticles these changes are more evident and can be related to the same effect. The bands shift in IR-spectra at 1000-1150 cm allows us... 

Figure I. Optical absorption spectra of the structures including PVC (a) and PEPC (b) films pure polymer films (1), films with Ag and Au nanopaiticles (2), films with thiol-stabilized Ag and Au nanoparticles (3).

Novel composite carbazole polymer films containing silver and gold nanoparticles and being thiol-stabilized have been fabricated. Optical properties of PEPC films are more sensitive to the nanoparticles incorporation than the properties of PVC films. This can be provided by the formation of additional interaction of particles and -CH2-O-CH2- group presented in the polymer chain. High values of the current density measured and the character of the current-voltage dependence in the composite films can be the consequence of several mechanisms of charge carrier transport in both composite PEPC and PVC films. 

In addition to chemical methods variety of physical methods has been employed for the synthesis of AuNPs. UV irradiation is used to improve the quality of the AuNPs when it is used in synergy with micelles or seeds [32,33], Near-IR laser irradiation provokes an enormous size growth of thiol-stabilized AuNPs [34], The presence of an ultrasonic field (200 kHz) allowed the control of the rate of AuC14" reduction in an aqueous solution containing only a small amount of 2-propanol and the sizes of the formed AuNPs are controlled by varying the parameters such as the temperature of the solution, the intensity of the ultrasound, and the positioning of the reactor [35,36], Sonochemistry was also used for the synthesis of AuNPs within the pores of silica and for the synthesis of Au/Pd bimetallic particles [37,38], Radiolysis has been used to control the size of AuNPs [39], Laser photolysis has been used to form AuNPs in block copolymer micelles. Laser ablation is another technique of AuNP synthesis that has been used under various conditions whereby size control can be induced by the laser [40,41],... 

Ab initio (HF, MP2, and MP4) (99JCS(P2)801) and SCF + MBTP(2) methods (90JPC7406) also predict the predominance of the thiol tautomer in the gas phase however, the relative stability of the thiol tautomer is overestimated by 1 kcal/mol and 6.5 kcal/mol, respectively. Similarly, calculations using large (TZV2P) basis sets and electron correlation at the QCISD(T) level overestimate the thiol stability by 3.6 kcal/mol (93JCS(P2)861). In contrast, the density functional theory (B3LYP)... 

Other methods for preparation of thiol-stabilized colloids have followed. Straightforward reduction of tetrachloroaurate can be performed in ethanol in the presence of an ethanol-soluble thiol329,483. Gold colloids can be prepared first by any conventional method, and then the colloid dispersion in water is mixed with the ethanol solution... 

Eychmuller, A. and Rogach, A. L. 2000. Chemistry and photophysics of thiol-stabilized II-VI semiconductor... 

Thiol stabilization also proved to offer a very successful route towards the creation of monosized nanoparticles of various compounds however, as it has also attracted widespread interest it will be outlined separately in Chapter 3.2.1.2. In this chapter, from this point onwards, attention will be focused on recent developments that differ from the hot injection synthesis and from the thiol technique. 

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