PACS. 73.30.+y Surface double layers, Schottky barriers, and work functions, 73.61.Ph Polymers; organic compounds,

Nanofluids (colloidal suspensions of nanoparticles) have been reported to display significantly enhanced thermal conductivities relative to those of conventional heat transfer fluids, also at low concentrations well below 1% per volume (Putnam, S. A., et at. J. Appl. Phys. 2006, 99, 084308; Liu, M.-S. L., et al. Int. J. Heat Mass Transfer. 2006, 49; Patel, H. E., et al. Appl. Phys. Lett. 2003, 83, 2931-2933). The purpose of this paper is to evaluate the effect of the particle size, concentration, stabilization method and particle clustering on the thermal conductivity of gold nanofluids. We synthesized spherical gold nanoparticles of different size (from 2 to 45 nm) and prepared stable gold colloids in the range of volume fraction of 0.00025-1%. The colloids were inspected by UV-visible spectroscopy, transmission electron microscope (TEM) and dynamic light scattering (DLS). The thermal conductivity has been measured by the transient hot-wire method (THW) and the steady state parallel plate method (GAP method). Despite a significant search in parameter space no significant anomalous enhancement of thermal conductivity was observed. The highest enhancement in thermal conductivity is 1.4% for 40 nm sized gold particles stabilized by EGMUDE (triethyleneglycolmono-11-mercaptoundecylether) and suspended in water with a particle-concentration of 0.11 vol%.

No abstract

Communications ADVANCED MATERIALSIt thus seems that the use of guest-host systems as active materials in LEDs constitutes a very simple yet powerful route towards optimization, for instance, in the direction of white light emission. One may control the emission properties independently of the nature of the chromophore by suitable selection of the matrix components. ExperimentalFilms for PL spectra measurements were prepared by spin-coating (2000 rpm) of polymerkhromophore solutions on quartz substrates. For systems with PN we used 1 wt.-% of polymerlPSA in methylene chloridelodichlorobenzene (ratio 1:9 or 1:l). All other systems were prepared from I wt.-% polymeriPSA in methylene chloride. To avoid impurities, we used solvents with HPLC grade. Later we precipitated all polymers in methanol. The last purification step was filtration of the polymerlchromophore solutions through a 2 Fm filter.Devices were prepared with the same solutions as described above by spin-coating (2000 rpm) onto IT0 substrates. A thin aluminum layer (typically 150 nm) was vacuum cvdporated ( P < 9 x 10" mbar) on top.The thickness of the active layers was determined with the Dektak3ST surface profiler lrom Veeco Instruments.Absorption spectra were recorded with a Perkin Elmer type Lambda 9. PL (excitation at 380 nm) and EL spectra were recorded with a Shimadzu type 1509. All characterization was performed at ambient atmosphere and temperature.

We have developed a simple method for the preparation of nearly mono-dispersed stable gold colloids with a fairly high concentration using a two step procedure. First we synthesize citrate capped gold nanoparticles and then exchange the citrate ions with triethyleneglycolmono-11-mercaptoundecylether (EGMUDE). This leads to the immediate precipitation and formation of composite assemblies. The gold nanoparticles were successfully self-redispersed after a few days. The prepared gold colloid can be easily concentrated up to 20 times by separation of the flocculated part. UV-visible spectra, transmission electron microscopy (TEM), and dynamic light scattering (DLS) were used to characterize the products thus formed.