Nanocrystalline materials form a new class of artificial solids. These solids have nanoparticles instead of atoms as building blocks. The ultimate goal of this work is to design novel materials with properties that are not found in natural solids.

Figure 1. High resolution transmission electron micrograph of a single gold nanocrystal. The alternating light and dark lines are atomic planes.

The first step is to create a nanocrystal building block. We use chemically synthesized gold nanocrystal that are highly uniform in size (5% or better). As we can see in Figure 1, an individual nanoparticle is crystalline and has a diameter of about 5.5 nm. The method for obtaining such uniform particles was developed by Xiao-Min Lin, Chris Sorensen and Ken Klabunde at Kansas State University. The next step is to assemble the nanoparticles into highly ordered patterns. Our goal has been to produce monolayers of particles in which there is near perfect long-range order, the particle spacing is less than 2 nanometers and is controlled to within a fraction of a nanometer, and in which the particles are fixed onto a solid, insulating substrate. An example of such monolayer on a silicon-nitride-covered silicon substrate is shown in Fig. 2. Remarkably, simple drop drying techniques are capable of producing this perfection. In fact, it appears that extended, highly ordered monolayers are comparatively difficult to lay down on solid sustrates with other methods.To get an idea of how our approach works, check out the animation here. Note that we drive the system far from equilibrium during the early stages of drop drying in order to move particles to the liquid-air interface.

Figure 2. Transmission electron microscope (TEM) pictures of a well-ordered array of gold nanocrystals. The top picture has been colorized. The bottom picture is an original b/w TEM image plus a Fourier transform. The different shades of red (top) or grey (bottom picture) of the particles come from different orientations of their atomic planes with respect to the incident electron beam.

To determine the electron transport properties of such a self-assembled monolayer, we use the drop drying technique to let the monolayer drape itself over a set of prefabricated electrodes on the substrate and, after the monolayer dried, measure the current as a function of applied voltage. Typical arrays are about 75 nanocrystals long and 300 nanocrystals wide for our electrode configuration. We find that the current is a nonlinear function of voltage. There is no current up to some voltage threshold, and above that the current is approximately quadratic function of the voltage (Fig. 3).

Figure 3. Current as a function of voltage for an array of nanocrystals.There is no current up to a certain voltage threshold. Above the voltage threshold the current increases nonlinearly with voltage.

What is the physics governing the electron transport through these tiny gold spheres? The nanocrystals are so small that if there is a single excess electron on them, then it is very difficult to add another electron. (The energy required is about 10-times the thermal energy at room temperature.) The energy comes from the electric potential of the electrode. Because we have to charge up a whole bunch of nanocrystals to get a current across the entire array, there is a threshold voltage below which no current is allowed to flow through the array. The nonlinear behavior after the voltage threshold arises because increasing voltage opens up more and more paths across the array.

A couple recent publications with more details:

X. M. Lin, H. M. Jaeger, C. M. Sorensen, and K. J. Klabunde, “Formation of Long-Range-Ordered Nanocrystal Superlattices on Silicon Nitride Substrates”, J. Phys. Chem. B 105, 3353 (2001).

Raghuveer Parthasarathy, Xiao-Min Lin, Thomas F. Rosenbaum and Heinrich M. Jaeger, “Electronic Transport in Metal Nanocrystals Arrays: The Effect of Structural Disorder on Scaling Behavior”, Phys. Rev. Lett. 87, 186807 (2001).

X.M. Lin, R. Parthasarathy, H.M. Jaeger, “Direct Patterning of Self-Assembled Nanocrystal Monolayers by Electron Beams”, Appl. Phys. Lett. 78, 1915 (2001).

Xiao-Min Lin, Raghuveer Parthasarathy and Heinrich M. Jaeger, “Self-Assembly and Physical Properties of Nanocrystal Arrays”, Encyclopedia of Nanoscience and Nanotechnology (Marcel Dekker, 2004). p. 2245-2258.

Raghuveer Parthasarathy, Xiao-Min Lin, Klara Elteto, T. F. Rosenbaum, and Heinrich M. Jaeger, “Percolating Through Networks of Random Thresholds: Finite Temperature Electron Tunneling in Metal Nanocrystal Arrays”, Phys. Rev. Lett. 92, 076801 (2004).

Klara Elteto, Eduard G. Antonyan, T.T. Nguyen, and Heinrich M. Jaeger , “A model for the onset of transport in systems with distributed thresholds for conduction”, Phys. Rev. B 71, 064206 (2005).

Klara Elteto, Xiao-Min Lin, and Heinrich M. Jaeger, “Fabrication and Transport Properties of Quasi-1D Arrays of Gold Nanocrystals”, submitted to Phys. Rev. B (2005) / cond-mat/0502675.