Gold nanoparticles (Au nanoparticles) that are ~12 nm in diameter were synthesized by rapidly injecting a solution of 150 mg (0.15 mmol) of tetrachloroauric acid in 3.0 g (3.7 mmol, 3.6 mL) of oleylamine (technical grade) and 3.0 mL of toluene into a boiling solution of 5.1 g (6.4 mmol, 8.7 mL) of oleylamine in 147 mL of toluene. While boiling and mixing the reaction solution for 2 hours, the color of the reaction mixture changed from clear, to light yellow, to light pink, and then slowly to dark red. The heat was then turned off, and the solution was allowed to gradually cool down to room temperature for 1 hour. The gold nanoparticles were then collected and separated from the solution using a centrifuge and washed three times; by vortexing and dispersing the gold nanoparticles in 10 mL portions of toluene, and then precipitating the gold nanoparticles by adding 40 mL portions of methanol and spinning them in a centrifuge. The solution was then decanted to remove any remaining byproducts and unreacted starting materials. Drying the gold nanoparticles in a vacuum environment produced a solid black pellet; which could be stored for long periods of time (up to one year) for later use, and then redissolved in organic solvents such as toluene.

The development of high efficiency thermoelectric materials would revolutionize energy harvesting capabilities and be useful for a large number of applications. Hybrid organic-inorganic nanostructured materials are an intriguing platform for developing efficient thermoelectric systems due to the possibility of independently controlling the thermal and electrical conductivity of the material with intelligent choices for material components. In this study we control the thermopower of hybrid 2-dimensional (2D) nanoparticle-molecule superlattices by systematically modifying molecular properties. Five conjugated, ladder-type, heteroacene molecules are used to interlink gold nanoparticles and control the thin films' properties. Interestingly, we measure a change in the sign of the Seebeck coefficient, corresponding to a crossover of the majority charge carrier (from hole to electron). Hall-effect measurements are used to confirm the change in dominant carrier for these systems. And density functional theory (DFT) is used in combination with Green's function-based transport calculations to examine the energy level-alignments in the system. The single-molecule Seebeck coefficient predictions from these results compare favorably with the experimental results of the molecular arrays and provide potential insights into the origins of the sign-change of the carriers in the system. In addition, the thermoelectric power factor sigma S-2 is found to range above predicted values for hybrid systems and to deviate from optimization strategies for conventional materials. A simple strategy to further increase sigma S-2 is highlighted. Limitations of the model and sources of variability in the experimental results are discussed. Our findings develop a stronger understanding of charge transport in molecule-nanoparticle hybrid films; demonstrate that these hybrid materials allow facile control over both the carrier type and the power factor of the material, both of which are important for maximizing the efficiency of functional thermoelectric devices; and establish a framework for continuing to maximize the thermoelectric efficiency of these materials.

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Nanogermanium is a material that has great potential for technological applications, and doped and alloyed Ge nanocrystals (NCs) are actively being considered. New alloys and compositions are possible in colloidal synthesis because the reactions are kinetically rather than thermodynamically controlled. Most of the Group V elements have been shown to be n-type dopants in Ge to increase carrier concentration; however, thermodynamically, Bi shows no solubility in crystalline Ge. Bi-doped Ge NCs were synthesized for the first time in a microwave-assisted solution route. The oleylamirie capping ligand can be replaced by dodecanethiol without loss of Bi. A positive correlation between the lattice parameter and the concentration of Bi content (0.5-2.0 mol %) is shown via powder X-ray diffraction and selected area electron diffraction. X-ray photoelectron spectroscopy, transmission electron microscopy (TEM), scanning TEM, and inductively coupled plasma-mass spectroscopy are consistent with the Bi solubility up to 2 mol %. The NC size increases with increasing amount of bismuth iodide employed in the reaction. Absorption data show that the band gap of the Bi-doped Ge NCs is consistent with the NC size. This work shows that a new element can be doped into Ge NCs via a microwave assisted route in amounts as high as 1-2 mol %, which leads to increased carriers. Colloidal chemistry provides an inroad to new materials not accessible via other means.

Controlling the initial ligand length determines the efficiency of the secondary ligand's substitution into the nanoparticle array. This determines post-exchange conductance in a manner akin to doping.

Composite molecule-nanoparticle hybrid systems have recently emerged as important materials for applications ranging from chemical sensing to nanoscale electronics. However, creating reproducible and repeatable composite materials with precise properties has remained one of the primary challenges to the implementation of these technologies. Understanding the sources of variation that dominate the assembly and transport behavior is essential for the advancement of nanoparticle-array based devices. In this work, we use a combination of charge-transport measurements, electron microscopy, and optical characterization techniques to determine the role of morphology and structure on the charge transport properties of 2-dimensional monolayer arrays of molecularly-interlinked Au nanoparticles. Using these techniques we are able to determine the role of both assembly-dependent and particle-dependent defects on the conductivities of the films. These results demonstrate that assembly processes dominate the dispersion of conductance values, while nanoparticle and ligand features dictate the mean value of the conductance. By performing a systematic study of the conductance of these arrays as a function of nanoparticle size we are able to extract the carrier mobility for specific molecular ligands. We show that nanoparticle polydispersity correlates with the void density in the array, and that because of this correlation it is possible to accurately determine the void density within the array directly from conductance measurements. These results demonstrate that conductance-based measurements can be used to accurately and non-destructively determine the morphological and structural properties of these hybrid arrays, and thus provide a characterization platform that helps move 2-dimensional nanoparticle arrays toward robust and reproducible electronic systems.

Liquid metal wires supported on substrates destabilize into droplets. The destabilization exhibits many characteristics of the Rayleigh-Plateau model of fluid jet breakup in vacuum. In either case, breakup is driven by unstable, varicose surface oscillations with wavelengths greater than the critical one (λ(c)). Here, by controlling the nanosecond liquid lifetime as well as stability of a rivulet as a function of its length by lithography, we demonstrate the ability to dictate the parallel assembly of wires and particles with precise placement.