Current Issue : April - June Volume : 2012 Issue Number : 2 Articles : 5 Articles
Nanoparticles of Ptââ?¬â??Rh were studied by means of lattice-based Monte Carlo simulations with respect to the stability of ordered\r\nD022- and 40-phases as a function of particle size and composition. By thermodynamic integration in the semi-grand canonical\r\nensemble, phase diagrams for particles with a diameter of 7.8 nm, 4.3 nm and 3.1 nm were obtained. Size-dependent trends such as\r\nthe lowering of the critical ordering temperature, the broadening of the compositional stability range of the ordered phases, and the\r\nnarrowing of the two-phase regions were observed and discussed in the context of complete size-dependent nanoparticle phase\r\ndiagrams. In addition, an ordered surface phase emerges at low temperatures and low platinum concentration. A decrease of platinum\r\nsurface segregation with increasing global platinum concentration was observed, when a second, ordered phase is formed\r\ninside the core of the particle. The orderââ?¬â??disorder transitions were analyzed in terms of the Warrenââ?¬â??Cowley short-range order parameters.\r\nConcentration-averaged short-range order parameters were used to remove the surface segregation bias of the conventional\r\nshort-range order parameters. Using this procedure, it was shown that the short-range order in the particles at high temperatures is\r\nbulk-like....
In recent years, self-assembled monolayers (SAMs) have been demonstrated to provide promising new approaches to nonlinear\r\nlaser processing. Most notably, because of their ultrathin nature, indirect excitation mechanisms can be exploited in order to fabricate\r\nsubwavelength structures. In photothermal processing, for example, microfocused lasers are used to locally heat the substrate\r\nsurface and initiate desorption or decomposition of the coating. Because of the strongly temperature-dependent desorption kinetics,\r\nthe overall process is highly nonlinear in the applied laser power. For this reason, subwavelength patterning is feasible employing\r\nordinary continuous-wave lasers. The lateral resolution, generally, depends on both the type of the organic monolayer and the\r\nnature of the substrate. In previous studies we reported on photothermal patterning of distinct types of SAMs on Si supports. In this\r\ncontribution, a systematic study on the impact of the substrate is presented. Alkanethiol SAMs on Au-coated glass and silicon\r\nsubstrates were patterned by using a microfocused laser beam at a wavelength of 532 nm. Temperature calculations and thermokinetic\r\nsimulations were carried out in order to clarify the processes that determine the performance of the patterning technique.\r\nBecause of the strongly temperature-dependent thermal conductivity of Si, surface-temperature profiles on Au/Si substrates are very\r\nnarrow ensuring a particularly high lateral resolution. At a 1/e spot diameter of 2 Ã?µm, fabrication of subwavelength structures with\r\ndiameters of 300ââ?¬â??400 nm is feasible. Rapid heat dissipation, though, requires high laser powers. In contrast, patterning of SAMs on\r\nAu/glass substrates is strongly affected by the largely distinct heat conduction within the Au film and in the glass support. This\r\nresults in broad surface temperature profiles. Hence, minimum structure sizes are larger when compared with respective values on\r\nAu/Si substrates. The required laser powers, though, are more than one order of magnitude lower. Also, the laser power needed for\r\npatterning decreases with decreasing Au layer thickness. These results demonstrate the impact of the substrate on the overall\r\npatterning process and provide new perspectives in photothermal laser patterning of ultrathin organic coatings....
The surface functionalization of inorganic nanostructures is an effective approach for enriching the potential applications of existing\r\nnanomaterials. Inorganic nanotubes attract great research interest due to their one-dimensional structure and reactive surfaces.\r\nIn this review paper, recent developments in surface functionalization of an aluminosilicate nanotube, ââ?¬Å?imogoliteââ?¬Â, are introduced.\r\nThe functionalization processes are based on the robust affinity between phosphate groups of organic molecules and the\r\naluminol (AlOH) surface of imogolite nanotubes. An aqueous modification process employing a water soluble ammonium\r\nsalt of alkyl phosphate led to chemisorption of molecules on imogolite at the nanotube level. Polymer-chain-grafted imogolite\r\nnanotubes were prepared through surface-initiated polymerization. In addition, the assembly of conjugated molecules, 2-(5ââ?¬â?¢Ã¢â?¬â?¢-hexyl-\r\n2,2ââ?¬â?¢:5ââ?¬â?¢,2ââ?¬â?¢Ã¢â?¬â?¢-terthiophen-5-yl)ethylphosphonic acid (HT3P) and 2-(5ââ?¬â?¢Ã¢â?¬â?¢-hexyl-2,2ââ?¬â?¢:5ââ?¬â?¢,2ââ?¬â?¢Ã¢â?¬â?¢-terthiophen-5-yl)ethylphosphonic acid 1,1-\r\ndioxide (HT3OP), on the imogolite nanotube surface was achieved by introducing a phosphonic acid group to the corresponding\r\nmolecules. The optical and photophysical properties of these conjugated-molecule-decorated imogolite nanotubes were characterized.\r\nMoreover, poly(3-hexylthiophene) (P3HT) chains were further hybridized with HT3P modified imogolite to form a nanofiber\r\nhybrid....
Background: The transport through a quantum-scale device may be uniquely characterized by its transmission eigenvalues n.\nRecently, highly conductive single-molecule junctions (SMJ) with multiple transport channels (i.e., several n > 0) have been\nformed from benzene molecules between Pt electrodes. Transport through these multichannel SMJs is a probe of both the bonding\nproperties at the leadââ?¬â??molecule interface and of the molecular symmetry.\nResults: We use a many-body theory that properly describes the complementary waveââ?¬â??particle nature of the electron to investigate\ntransport in an ensemble of Ptââ?¬â??benzeneââ?¬â??Pt junctions. We utilize an effective-field theory of interacting -electrons to accurately\nmodel the electrostatic influence of the leads, and we develop an ab initio tunneling model to describe the details of the leadââ?¬â??molecule\nbonding over an ensemble of junction geometries. We also develop a simple decomposition of transmission eigenchannels into\nmolecular resonances based on the isolated resonance approximation, which helps to illustrate the workings of our many-body\ntheory, and facilitates unambiguous interpretation of transmission spectra.\nConclusion: We confirm that Ptââ?¬â??benzeneââ?¬â??Pt junctions have two dominant transmission channels, with only a small contribution\nfrom a third channel with n << 1. In addition, we demonstrate that the isolated resonance approximation is extremely accurate and\ndetermine that transport occurs predominantly via the HOMO orbital in Ptââ?¬â??benzeneââ?¬â??Pt junctions. Finally, we show that the transport\noccurs in a leadââ?¬â??molecule coupling regime where the charge carriers are both particle-like and wave-like simultaneously,\nrequiring a many-body description....
Self-assembled monolayers (SAMs) of nitrile-substituted oligo(phenylene ethynylene) thiols (NC-OPEn) with a variable chain\r\nlength n (n ranging from one to three structural units) on Au(111) were studied by synchrotron-based high-resolution X-ray photoelectron\r\nspectroscopy and near-edge absorption fine-structure spectroscopy. The experimental data suggest that the NC-OPEn\r\nmolecules form well-defined SAMs on Au(111), with all the molecules bound to the substrate through the goldââ?¬â??thiolate anchor and\r\nthe nitrile tail groups located at the SAMââ?¬â??ambient interface. The packing density in these SAMs was found to be close to that of\r\nalkanethiolate monolayers on Au(111), independent of the chain length. Similar behavior was found for the molecular inclination,\r\nwith an average tilt angle of ~33ââ?¬â??36Ã?° for all the target systems. In contrast, the average twist of the OPEn backbone (planar conformation)\r\nwas found to depend on the molecular length, being close to 45Ã?° for the films comprising the short OPE chains and\r\n~53.5Ã?° for the long chains. Analysis of the data suggests that the attachment of the nitrile moiety, which served as a spectroscopic\r\nmarker group, to the OPEn backbone did not significantly affect the molecular orientation in the SAMs....
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