Two types of electronic charge transport in metal nanoparticle arrays: sequential tunneling and inelastic cotunneling

We investigate transport in weakly coupled metal nanoparticle arrays, focusing on the regime where tunneling is competing with strong single electron charging effects. This competition gives rise to an interplay between two types of charge transport. In sequential tunneling, transport is dominated by independent electron hops from a particle to its nearest neighbor along the current path. In inelastic cotunneling, transport is dominated by cooperative multielectron hops that each go to the nearest neighbor but are synchronized to move charge over distances of several particles. In order to test how the temperature-dependent cotunnel distance affects the current-voltage I-V characteristics, we perform a series of systematic experiments on highly ordered close-packed nanoparticle arrays. The arrays consist of ~5.5 nm diameter gold nanocrystals with tight size dispersion, spaced ~1.7 nm apart by interdigitating shells of dodecanethiol ligands. We present I-V data for monolayer, bilayer, trilayer, and tetralayer arrays. For stacks 2–4 layers thick we compare in-plane measurements with data for vertical transport perpendicular to the array plane.

Freestanding elastic membranes of close-packed nanoparticle arrays

Nanoparticle superlattices are hybrid materials composed of close-packed inorganic particles separated by short organic spacers. Most work so far has concentrated on the unique electronic, optical and magnetic behaviour of these systems. Here, we demonstrate that they also possess remarkable mechanical properties. We focus on two-dimensional arrays of close-packed nanoparticles and show that they can be stretched across micrometre-size holes. The resulting freestanding monolayer membranes extend over hundreds of particle diameters without crosslinking of the ligands or further embedding in polymer. To characterize the membranes we measured elastic properties with force microscopy and determined the array structure using transmission electron microscopy. For dodecanethiol-ligated 6-nm-diameter gold nanocrystal monolayers, we find a Young’s modulus of the order of several GPa(*). This remarkable strength is coupled with high flexibility, enabling the membranes to bend easily while draping over edges. The arrays remain intact and able to withstand tensile stresses up to temperatures around 370 K. The purely elastic response of these ultrathin membranes, coupledwith exceptional robustness and resilience at high temperatures should make them excellent candidates for a wide range of sensor applications. Read more in the press: The University of Chicago News Office, Physorg.com, nanotechwire.com, ScienceDaily,...

* note: using a better fitting procedure we report slightly revised Young's moduli in He et al., Small, 2010 (above).

Dried to Perfection: Ordered monolayer nanoparticle arrays produced with a simple drop drying technique

When a drop of colloidal solution of nanoparticles dries on a surface it leaves behind coffee-stain-like rings of material with lace-like patterns or clumps of particles in the interior..These non-uniform mass distributions are manifestations of far-from-equilibrium effects such as fluid flows and solvent fluctuations during late-stage drying. Recent experiments, however, have found a strikingly different drying regime promising highly-uniform, long-range-ordered nanocrystal monolayers. We have made direct, real-time and real-space observations of colloidal nanocrystal self-assembly to reveal the mechanism. We show how the morphology of drop-deposited nanoparticle films can be controlled by rapid evaporation kinetics and particle interactions with the liquid-air interface. This growth mode leads to exceptional long-range ordering, despite evaporation occurring far from equilibrium, and ultimately forms compact monolayers that cover the entire macroscopic surface. Read more...(nugget, high res images; plus: technical details, movies)