Weizmann Institute scientists have made an important step towards harnessing organic molecules to future electronics. Reported in the March 9th issue of Nature, their approach places common semiconductor-based devices -- for the first time ever -- under the control of organic molecules.
The functions of organic molecules are so diverse that their inclusion in electronics would provide an extensive range of possibilities. However, the observation of these molecules' electrical properties has up until now been impeded by incongruities in the structure of organic molecules themselves. Layers of organic molecules that are used in this kind of research contain 'pinholes' -- small defects that are very difficult to detect but radically sway conductance. Scientists were unable to determine whether their measurements resulted from the passage of the current through the organic molecules or through a pinhole. But the new approach circumvents this problem.
The Weizmann scientists chose to analyze the molecules indirectly -- by focusing on the influence that the molecules were suspected to have on semiconductors. Using a series of molecules synthesized by Prof. Abraham Shanzer of the Weizmann Institute's Organic Chemistry Department, Ayelet Vilan, a graduate student working with Prof. David Cahen of the Materials and Interfaces Department, constructed a one-molecule-thick layer (monolayer) of very short organic molecules.
Vilan placed the monolayer on a common semiconductor, GaAs, and directed an electric current through it. The monolayer was so thin that, for the most part, the electric current passed by the molecules without interacting with them. This fact meant that it was of minimal importance if the electrons went via a molecule or a pinhole. (However, it is important to note that while the organic molecules barely affect the passage of the electrical current through them, they very much influence the electric properties of the semiconductor.)
The decision to work with monolayers of organic molecules compelled Vilan to develop a new method for preparing semiconductor devices. The technique is founded on a widely used semiconductor device (diode), which is comprised of a semiconductor connected to a metal. She inserted the organic monolayer between these two components. Since the organic molecules were 'sandwiched' between the semiconductor and the metal sheet, it was critically important to ensure that the delicate monolayer would not be crushed underneath the metal sheet. Vilan, building on the findings of Ellen Moons, one of Cahen's former students, reworked a method used in other fields to suit the device. She used a thin gold leaf as the metal sheet and gently floated it onto the monolayer. Thus, the monolayer remained intact.
The scientists found that changing the type of organic molecules in a monolayer led to a predictable, systematic change in the electrical characteristics of the device. Thus, not only were they able to control the properties of the semiconductor, but they also were able to predict the kind of control that would be exerted by different types of organic molecules.
"This study introduces a feasible way to incorporate organic molecules into electronic devices," says Vilan. "But mainly, it provides new insights into the emerging field of molecular electronics. So little is known about the effects that occur between molecules and the electric conductors we normally use. This approach may provide a basis for the design of novel types of semiconductor-based devices, from improvements in relatively simple devices such as solar cells, to possible new types of computer chips."
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