July 22, 2005
When contemplating an ancient Greek mosaic, one sometimes experiences a sense of the profound. The artistic impact of the whole contrasts markedly with that of each individual glass shard, pebble, or tile fragment composing it, although each element contributes to the overall effect.
Professor Heinrich Jaeger and his collaborators at the Chicago Materials Research Center and Dr. Xiao-Min Lin of Argonne National Laboratory have been actively engaged in a research direction that may also elicit a similar sense of the profound. Through this research, they have obtained evidence of an additional mechanism for the movement of electrical current through an “artificial solid” consisting of a 2D-lattice of gold nanocrystals.
Artificial solids, in general, are constructed by specifically assembling a number of nanocrystals (each composed of only a few thousand atoms) into a closely-packed and well-ordered lattice. By changing the composition and/or size of the nanocrystals, specialty materials can be developed that exhibit novel electronic, magnetic, and optical properties. In these materials, each nanocrystal acts as a single atom in a lattice site, hence the description “artificial solid.” Analogously to the Greek mosaic—although one needs a powerful microscope to see these—the cooperative behavior of the solid, taken as a whole, contrasts with the individual behavior of each constitutive element.
In the Jaeger lab, the researchers created well-ordered arrays (Figure 1a,b) by depositing single layers of ~5 nm in diameter gold nanocrystals onto a special surface in the presence of a slight excess of 1-dodecanethiol (a long organic molecule which binds selectively--at only one end-- to gold surfaces) [1]. Through techniques developed in their laboratory, they delineated an area of each array for experimentation, and sandwiched it between a pair of planar electrodes (Figure 2). Applying a range of voltages across the electrodes, they monitored the amount of current produced [2,3,4].
Plotting the current produced at each applied voltage, the researchers observed non-linear curves whose general features are readily explained by earlier studies. For example, the presence of a definite threshold voltage implies that current travels from nanocrystal to nanocrystal. Because the addition of electrons to each nanocrystal requires a certain amount of energy, no current flow (i.e. movement of electrons) will be observed until that amount of energy (i.e. threshold voltage) is applied . Also, the specific threshold voltage observed depends upon the length of the array (i.e. how many nanocrystals) between the electrodes , and significant deviations from expectations can be attributed to the presence of regions of structural disorder in the array [2].
At lower temperatures (under 100 K), the researchers noticed an unexpected trend. They observed that the threshold voltage shifts to lower values and varies linearly, with increasing temperature (Figure 3). This suggests that for these temperatures, another pathway--a shortcut, if you will-- plays a decided role in the ability of the electron to migrate through the nanocrystalline array. The underlying lattice of 1-docanethiol ligands, which initially assists in the formation of the well-ordered nanocrytalline network by regulating solubility and spacing, also assists with the transfer of electrical charge onto and between nanocrystals, thus lowering the overall energy requirement [3,5].
Although artificial solids are expected to revolutionize the fabrication of electronic devices in the near future, further understanding of their fundamental behavior is necessary before they can actually be used. Through careful experimentation, the Jaeger lab has uncovered a significant part of the puzzle.
by Eileen Sheu, Klara Elteto
References:
- "Formation of Highly Long-Range-Ordered Nanocrystal Superlattices on Silicon Nitride Substrates," X.-M. Lin, H.M. Jaeger, C.M. Sorensen, and K.J. Klabunde, J. Phys. Chem. B,105, 3353 (2001)
- "Electronic Transport in Metal Nanocrystal Arrays: The Effect of Structural Disorder on Scaling Behavior," R. Parthasarathy, X.-M. Lin, and H.M. Jaeger, Phys. Rev. Lett. 87, 186807 (2001).
- "Percolating through Networks of Random Thresholds: Finite Temperature Electron Tunneling in Metal Nanocrystal Arrays," Raghuveer Parthasarathy, Xiao-Min Lin, Klara Elteto, Thomas F. Rosenbaum, and Heinrich M. Jaeger, Physical Review Letters, 92, 076801, (2004).
- "Electronic transport in quasi-one-dimensional arrays of gold nanocrystals," Klara Elteto, Xiao-Min Lin, and H.M. Jaeger,Phys. Rev. B, 71, 05412 (2005).
- "Model for the onset of transport in systems with distributed thresholds for conduction," Klara Elteto, Eduard G. Antonyan, T.T. Nguyen, and Heinrich M. Jaeger, Phys. Rev. B, 71, 064206 (2005).