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ScienceWeek
MOLECULAR ELECTRONICS: ON MOLECULAR JUNCTIONS
The following points are made by Mark Ratner (Nature 2005 435:575):
1) In its simplest form, a molecular junction consists of a single organic molecule sandwiched between two much larger electrodes. More efficient and more precise control of current flow through such elements, and a detailed understanding of the factors that influence that flow, are essential to take electronics from the microscale of conventional, silicon-based technology down to the nanoscale. New work [1] demonstrates how to control the onset of conduction through a molecular junction by using the charge state of an atom on the surface of a silicon electrode.
2) The difficulty with structures such as molecular junctions is that although the molecule consists of a series of discrete states in a small finite entity, the electrodes contain a very dense set of states in a macroscopic structure. Understanding how the electrostatic environment of the molecule modifies the transport process -- in other words, the influence of the electrode -- is in many ways the most vexing problem in dealing with such junctions.
3) In the more typical metal-molecule-metal type of molecular junction [2-5], extra complexity results from the geometric disorder inherent in the usual sorts of metal-molecule coordination bonding. Typically, these bonds are between a coinage metal (copper, silver or gold) and a thiol group, or between palladium and a cyano group. Geometric changes can strongly affect charge transport through these junctions and give rise to the random switching phenomena often seen in such structures.
4) One way to avoid this geometric uncertainty is to exploit transport in junctions built not on a metal but on a semiconductor (particularly silicon), where the molecule-electrode interface can be provided by two atoms "sharing" the electrons in a covalent bond. This bond can be created in several ways12, but perhaps most directly by linking a free radical at the end of the molecule -- in this case, a carbon atom with an unpaired electron -- to a "dangling bond" on the surface of the silicon electrode. This dangling bond can arise through the removal of a hydrogen atom originally attached to each silicon atom at the surface of the passivated electrode, leaving behind an unpaired electron that hangs free.
5) Wolkow et al [1] took advantage of two remarkable properties of silicon surfaces to characterize how changes in the charge state of a silicon surface atom influence the effective field over the molecular junction. First, the "polymerization" of the molecules -- the process by which they attach themselves covalently along "dimer rows" on the silicon surface to form a line -- ends abruptly at a dangling-bond site. Second, the dangling-bond site itself can become more or less charged depending on the doping level of the silicon (that is, the deficiency or excess of electrons that is induced by adding an "impurity element" with an intrinsic number of valence electrons different from silicon's four).
6) Wolkow et al [1] used a scanning-probe microscope to examine the effective charges along the length of the polymers originating from a given dangling-bond site. They found that when the dangling-bond site is charged, a "slope" structure is seen that is absent when the site is uncharged: molecules attached to the silicon electrode close to a charged dangling bond appear to stand out farther from the electrode than those at a greater distance from the dangling-bond site. Wolkow et al interpret this as the effect of a local electrostatic charge on charge transport through the polymeric wire. This interpretation is supported by calculations showing changes in molecular orbital states caused by the charged dangling-bond site. This result constitutes direct evidence that localized charges profoundly affect charge transport in single-molecule structures on silicon surfaces at room temperature.
References (abridged):
1. Piva, P. G. et al. Nature 435, 658-661 (2005)
2. Bowler, D. R. J. Phys. Condens. Matter 16, R721-R754 (2004)
3. Nitzan, A. & Ratner, M. A. Science 300, 1384-1389 (2003)
4. Reed, M. A. & Takhee, L. (eds) Molecular Nanoelectronics (American Scientific, Stevenson Ranch, CA, 2003)
5. Nitzan, A. Annu. Rev. Phys. Chem. 52, 681-750 (2001)
Nature http://www.nature.com/nature
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CHEMISTRY: PROBLEMS IN MOLECULAR ELECTRONICS
The following points are made by A.H. Flood et al (Science 2004 306:2055):
1) The drive toward yet further miniaturization of silicon-based electronics has led to a revival of efforts to build devices with molecular-scale components. The field of molecular electronics is teeming with results, rationalizations, and speculations. Some claims may have been exaggerated, but news stories of a crisis in the field [1] are premature. Reports of passive molecular electronics devices, such as tunnel junctions and rectifiers, as well as of active devices, for example, single-molecule transistors and molecular switch tunnel junctions, have withstood scientific scrutiny.
2) Simple molecular electronic devices usually consist of organic molecules sandwiched between conducting electrodes. According to early predictions, such devices could show electron tunneling [2) or one-way flow of current (rectification) [HN3] through the molecule [3]. In most tunneling junctions, linear alkanes are sandwiched between metal electrodes. Measurements over the past 25 years [4,5] have largely validated McConnell's prediction [2] that the tunnel current depends exponentially on the length of the molecules between conducting electrodes. In rectifiers, a molecule composed of an electron donor, a bridge, and an electron acceptor is extended between two electrodes. Experiments have again validated the early prediction by Aviram and Ratner [3].
3) However, for both types of devices, the problem has been in the details, and reaching agreement between experiments and theory has not been straightforward. In the case of tunnel junctions, McConnell's prediction breaks down for alkanes with more than about 16 carbon atoms in the chain, because coherent tunneling is replaced by diffusive charge transport in longer chains. Furthermore, in all devices, the molecules tilt at an angle smaller than 90 deg with respect to the electrode surfaces. This angle -- and hence the separation between the electrodes --varies across different device constructions. Such variations can affect the measured current levels and can also dictate at which alkane chain length diffusive transport replaces tunneling. Other issues, such as the choice of electrode materials, can have similar effects.
4) In the case of rectifiers, it has turned out to be relatively easy to observe rectification, but nontrivial to observe true molecular rectification. This problem arises because current can be rectified in many parts of the device -- for example, at the molecule/electrode interfaces. True molecular rectification is observed only if the donor-bridge-acceptor component of the molecule is extended between the electrodes, and for only a relatively small range of donor and acceptor molecular orbital energy levels. Thus, strict attention to the molecular components, and to the molecule/electrode interfaces, is required.
References (abridged):
1. R. F. Service, Science 302, 556 (2003)
2. H. McConnell, J. Chem. Phys. 35, 508 (1961)
3. A. Aviram, M. A. Ratner, Chem. Phys. Lett. 29, 277 (1974)
4. E. E. Polymeropoulos, J. Sagiv, J. Chem. Phys. 69, 1836 (1978)
5. T. Lee et al., J. Phys. Chem. B 108, 8742 (2004)
Science http://www.sciencemag.org
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ON MOLECULAR ELECTRONICS
The following points are made by J.R. Heath and M.A. Ratner Physics Today 2003 May)
1) Molecules have not historically played a prominent role in electronic devices. Ten years ago, chemical applications were limited to the use of small molecules such as silanes and germanes as thin-film precursors or as the components of etching processes, resist precursors, packaging materials, and the like. Engineered inorganic insulators, semiconductors, and metals were the heart of the industry, and the fundamental knowledge that gave birth to the integrated circuit was appropriately credited back to the fundamental solid-state physics that was largely developed in the mid-20th century.
2) Over the past decade, the picture has not changed much. Conducting polymers have emerged as a real, albeit still minor, technology. However, over the next 10-20 years, molecules may be increasingly viewed not just as the starting points for bulk electronic materials, but as the active device components within electronic circuitry. Although this possibility is hardly a foregone conclusion, a number of fundamental issues favor the development of a true molecular-based electronics.
3) Consider a lattice of nanowires. Each wire is 5 nm in diameter, and the lattice constant is 15 nm. At a typical doping level of 10^(18) atoms of boron or arsenic per cubic centimeter, similar 5-nm diameter, micron-long segments of silicon wires would have 15-20 dopant atoms, and a junction of two crossed wires would contain, on average, approximately 0.1 dopant atom. Consequently, field-effect transistors fabricated at these wiring densities might exhibit non-statistical and perhaps unpredictable behavior. Other concerns, such as the gate oxide thickness, power consumption (just from leakage currents through the gate oxide), and fabrication costs, also highlight the difficulty of scaling standard electronics materials to molecular dimensions. At device areas of a few tens of square nanometers, molecules have an inherent attractiveness because of their size, because they represent the ultimate in terms of atomic control over physical properties, and because of the diversity of properties -- such as switching, dynamic organization, and recognition -- that can be achieved through such control.
4) Although molecular electronics has been the subject of research for some time, over the past few years a number of synthetic and quantum chemists, physicists, engineers, and other researchers have sharply increased the ranks of this field. Several new molecular-electronic systems, analytical tools, and device architectures have been introduced and explored. As a result, the basic science on which a molecular electronics technology would be built is now unfolding. For example, current research is using molecules in such electronics applications as interconnects, switches, rectifiers, transistors, nonlinear components, dielectrics, photovoltaics, and memories.
Physics Today http://www.physicstoday.org
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