August 28, 2003
Yu’s fundamental strategy was to form a molecular material containing two distinct parts by coupling an electron-rich oligo-thiophene segment (shown in blue-- note that the rings only contain "S") with an electron-poor oligo-thiazole segment (shown in red-- note that the rings contain both "N" and "S"), which have been demonstrated to be effective hole and electron transporting materials, respectively. The molecular structure then exhibits a built-in electronic asymmetry, resembling the semiconductor p–n junction. Initially, different alkyl side chains (the zig-zag arms extending from the middle of the picture to the right) with contrasting affinities for water, were added to the segments to make the molecules congregate at the water/solvent interface , from whence they can be transferred to a single layer of molecules on a solid surface.
Electrical properties of this diblock molecular material were determined using scanning tunneling spectroscopy (STS). For the diblock thiazole-thiophene compound, an asymmetric I-V (current vs. voltage) curve was measured, with turn-on voltage occurring at around +1.0 V. This asymmetry means that the current passes much more easily forward than backward, the novel property described above. The same measurements on an analogous thiophene-thiophene (“mono-block”) compound, which was synthesized for comparison, resulted in a nearly symmetric I-V curve and a much smaller current.
Wishing to examine the rectifying effect of the fundamental unit of this system, the diblock oligomer without long alkyl side chains, these molecules were inserted into a SAM (self-assembled monolayer) of alkanethiols on a gold surface, for electrical measurements. Areas where the molecules were incorporated appeared as bright spots upon STM (scanning tunnelling microscopy) imaging. (see inset left). An asymmetric I-V curve, characteristic of the molecular diode, was obtained. Similarly, measuring the electrical properties of the “monoblock” oligomer (also without side-chains) inserted into a SAM, also resulted in the expected symmetric I-V curve.
These rectifying conjugated molecules provide an easy entry to molecular-scale electronic components for the design of logic circuits. A large number of structural and electronic property variations based on this diblock system, can be readily envisioned, resulting in "made-to-order diodes."
by Eileen Sheu, Thomas Witten
- Man-Kit Ng, Dong-Chan Lee and Luping Yu, J. Am. Chem. Soc. 124 (40), 11862-11863 (2002).
- Man-Kit Ng and Luping Yu, Angew. Chemie, Int. Ed., 41, (19), 3598-3601, (2002).