No abstract

Ordered arrays, or superlattices, of metallic, insulating, or semiconducting quantum dots, represent an exciting new class of materials. These superlattices are often referred to as artificial solids, in which the nanocrystals take the place of atoms in traditional solids, and the packing arrangement of the nanocrystals determines the unit cell parameters of the superstructure. In this review, we discuss various approaches toward assembling nanocrystal superlattices and we discuss their physical properties.

Gold nanoparticle/alkanedithiol films were prepared via layer-by-layer self-assembly. For the assembly process, dodecylamine-stabilized Au nanoparticles with an average size of 4 nm and alkanedithiols with different alkylene chain lengths (C 6 , C 9 , C 12 , C 16 ) were used. The thickness of the films was determined by AFM and ranged between 26 and 34 nm. FE-SEM and TEM images indicate that the particle size within the film materials was similar to that of the dodecylamine-stabilized particles used for film preparation. The composition of the films was analyzed by XPS. The absence of the nitrogen signal indicated that the dodecylamine ligands were quantitatively exchanged by alkanedithiol molecules during film assembly. Two sulfur signals were observed, which could be assigned to sulfur bound to gold (S-Au) and to free thiol groups (S-H). As indicated by the relative signal intensities, about 60% of the alkanedithiol molecules were bound with both ends to the nanoparticles, whereas 40% were bound with only one thiol group. The C/S ratio was in good agreement with the stoichiometry of the alkanedithiol molecules. All films showed linear current-voltage characteristics. Conductivity measurements at variable temperature were consistent with an Arrhenius-type activation of charge transport. Using an activated tunneling model for describing the charge transport properties, we obtained an electron tunneling decay constant of β N ) 0.61 or 0.71, depending on the method used for data analysis. When the films were dosed with vapors of toluene and tetrachloroethylene, the resistance of the films increased reversibly. This response increased exponentially with increasing length of the alkanedithiol molecules. The chemical selectivity of the films corresponded essentially to the solubility properties of the alkanedithiol molecules.

Vapor-sensitive thin-film resistors comprising gold nanoparticles and different types of organic dendrimers (polyphenylene, poly(propylene imine) and poly(amidoamine)) were prepared via layer-by-layer self-assembly and characterized by UV/vis spectroscopy, atomic force microscopy, and conductivity measurements. While the metal nanoparticles were utilized to provide the film material with electric conductivity, the dendrimers served to cross-link the nanoparticles and to provide sites for the selective sorption of analyte molecules. Dosing the films with vapors of toluene, 1-propanol, and water significantly increased the film resistances. The chemical selectivity of this response was controlled by the solubility properties of the dendrimers.

The optical and electrical properties of 11-20 nm thick films composed of approximately 4 nm gold nanoparticles (Au-NPs) interlinked by six organic dithiol or bis-dithiocarbamate derivatives were compared to investigate how these properties depend on the core of the linker molecule (benzene or cyclohexane) and its metal-binding substituents (thiol or dithiocarbamate). Films prepared with the thiol-terminated linker molecules, (1,4-bis(mercaptomethyl)benzene, 1,4-bis(mercaptomethyl)cyclohexane, 1,4-bis(mercaptoacetamido)benzene, and 1,4-bis(mercaptoacetamido)cyclohexane), exhibit thermally activated charge transport. The activation energies lie between 59 and 71 meV. These films show distinct plasmon absorption bands with maxima between 554 and 589 nm. In contrast, the film prepared with 1,4-cyclohexane-bis(dithiocarbamate) has a significantly red-shifted plasmon band ( approximately 626 nm) and a pronounced absorbance in the near infrared. The activation energy for charge transport is only 14 meV. These differences are explained in terms of the formation of a resonant state at the interface due to overlap of the molecular orbital and metal wave function, leading to an apparent increase in NP diameter. The film prepared with 1,4-phenylene-bis(dithiocarbamate) exhibits metallic properties, indicating the full extension of the electron wave function between interlinked NPs. In all cases, the replacement of the benzene ring with a cyclohexane ring in the center of the linker molecule leads to a 1 order of magnitude decrease in conductivity. A linear relationship is obtained when the logarithm of conductivity is plotted as a function of the number of nonconjugated bonds in the linker molecules. This suggests that nonresonant tunneling along the nonconjugated parts of the molecule governs the electron tunneling decay constant (beta(N)(-)(CON)), while the contribution from the conjugated parts of the molecule is weak (corresponding to resonant tunneling). The obtained value for beta(N)(-)(CON) is approximately 1.0 (per non-conjugated bond) and independent of the nanoparticle-binding group. Hence, the molecules can be viewed as consisting of serial connections of electrically insulating (nonconjugated) and conductive (conjugated) parts.