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Electron transport across molecules:
From well-defined monolayers of simple alkyls to proteins #
High quality semiconductor/alkyl chain monolayers/metal structures, with the molecules chemically bound to the semiconductor (Si, GaAs) and/or the metal (Hg, Au, Pd) give reproducible and reliable data for electronic transport across molecules. Combining transport results with those from photoemission (Kahn, Ueno) and electronic structure calculations (Kronik) lead us to question some “conventional wisdoms”:
* To what extent should we use the simple HOMO/LUMO concept for these systems ?
* How relevant is the concept of a transport barrier with a well-defined energy & width?
* How important is molecule-electrode chemical bonding for transport?
and, more trivial(?)
* Do we actually know what are the contact areas in molecular electronics?
In spite of these uncertainties, our work with these simple systems showed a way to work with much more complicated biological ones. I'll illustrate the latter with results on bacteriorhodopsin. 3 While much more work is needed, results already hold some surprises.
# with A. Salomon, O. Seitz, H. Shpaisman, F. Thieblemont, R. Har-Lavan, G. Nesher, A. Vilan, I. Ron, Y.-D. Jin, and with the groups of L. Kronik and M. Sheves, Weizmann Inst , A. Kahn, Princeton Un., E. Umbach, Würzburg Un., Germany, and N. Ueno, Chiba Un., Japan and with T. Boecking & J. Gooding , UNSW, Sydney, Australia
References
D. Cahen et al., Mater. Today, 8 (2005) 32; A. Salomon et al., Phys. Rev. Lett. 95 (2005) 266807 , Nano. Lett . 2006, 6, 2873; Adv. Mater. 19 (2007) 445-450 ; O. Seitz et al. Langmuir, 22 (2006) 6915; JACS 129 (2007) 7494; G. Nesher et al., J. Phys. Chem. B, 110 (2006) 14363; JACS 129 (2007) 734 L. Segev et al.; Phys. Rev. B 74 (2006) 165323,; F. Amy et al., J. Phys. Chem. B ., 110 (2006) 21826; Y. Jin et al., PNAS 103 (2006) 8601.
Biography
David Cahen completed his B.Sc. in chemistry and physics at the Hebrew University of Jerusalem, and his Ph.D. in (Materials) Chemistry at Northwestern Univ, where he was one of the first to actually bridge disciplines, departments and faculties by working with Jim Ibers and Bruce Wagner, as well as independently. His research concentrated on comparing the effects of mixed valence in molecular and non-molecular solid state materials.
During this time he spent half a year at Stanford, joining the interdisciplinary search for high-temperature superconductors, working with Bill Little, Jim Collman and Dick Bube. After his return to Israel, his postdoctoral research was in photosynthesis, elucidating development of photosynthetic activity in photosynthetic algae. In 1976 he joined the Weizmann Institute in the area of energy conversion, elucidating energy balance and loss mechanisms of natural and artificial photoconversion systems, from photosynthetic bacteria and green plants to photoelectrochemical solar cells.
This led to work on the chemical aspects of electronic materials and devices and to determining fundamental chemical limits to device miniaturization and device stability. In parallel he explored how and when defects in materials can actually improve material quality and device performance, an area in which he remains active today for poly- and nano-crystalline solar cells. These interests led to pioneering work on hybrid molecular/non-molecular materials, which, in turn, evolved into his present activities. From the more practical point of view he showed how organic molecules can fine-tune the performance of inorganic solar cells and how the hybrid approach leads to a new type of sensor.
In terms of basic understanding he has shown that molecules can control electronic transport, either by electron transport across molecules or by "action at a distance" (molecular "doormen") and that the chemistry of molecular/non-molecular interfaces can totally dominate their performance as electronic contacts. His work focuses on understanding the limitations of such systems and exploring the possibilities for fundamentally novel science.
To read more about Dr. Cahen, please click here.
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