August 28, 2003
As the state-of-the-art in electronics technology improves, our computers, digital cameras, and cell phones are rapidly shrinking. The limit to which we can eventually miniaturize our electronics equipment depends primarily on our ability to develop smaller and smaller electronic components that are easily adaptable, cheap to manufacture, and which work reliably.
Professor Luping Yu and his collaborators at the Materials Research Center have made a significant contribution towards this goal of miniaturization by synthesizing and physically characterizing the first nanoscale polymeric molecular system that behaves similarly to a semiconductor p-n junction diode. Diodes (also known as rectifiers), in general, allow electrical current to flow forward but not backward in a circuit, and are typically constructed by joining p-type and n-type doped semiconductors, made from silicon or germanium wafers. (They are key components of electrical circuitry, commonly used to form switches, transistors, etc. and can be purchased from your local electronics supply store.) Professor Yu’s molecular diodes represent a leap in design, however, because not only are they synthesized chemically which makes them amenable to structural variation, thus easily tunable, they also have dimensions on the order of nanometers (10-9m)—more than a million times smaller than those presently available commercially.
Electrons flow along the central line of the double and single bonds of the chemical species in the middle. The alkyl side-chains (zig-zags) are placed on the molecule to make the blue part soluble in water and the red part insoluble in water. In the actual molecule the bonds can rotate, so these side-chains are flexible and randomly oriented.
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
References:
- 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).