PerspectiveChemistry

Whence Molecular Electronics?

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Science  17 Dec 2004:
Vol. 306, Issue 5704, pp. 2055-2056
DOI: 10.1126/science.1106195

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 [HN1]. 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. Simple molecular electronic devices usually consist of organic molecules sandwiched between conducting electrodes. According to early predictions, such devices could show electron tunneling (2) [HN2] 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 [HN4]. In rectifiers, a molecule composed of an electron donor, a bridge, and an electron acceptor is extended between two electrodes (see the first figure, top panel). Experiments (6, 7) have again validated the early prediction by Aviram and Ratner (3) [HN5].

Molecular electronic devices.

(Top) A molecular rectifier constructed from donor (red), bridge (black), and acceptor (blue) components. (Middle) A single-molecule transistor constructed from a symmetrical cobalt complex. S, source; D, drain; G, gate. (Bottom) A molecular switch tunnel junction in its Off and On states. (Left) Structural formula of the On state of a bistable [2]rotaxane.

However, for both types of devices, the devil 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 [HN6] 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° 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.

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.

Active molecular electronic devices include single-molecule transistors [HN7] and molecular switch tunnel junctions. The development of these more complex devices has been guided by experiment rather than theory. To date, only a couple of systems have passed scientific scrutiny from multiple laboratories. To validate such devices, one compelling approach has been to identify unique properties that can be observed and quantified in both the devices and in solution.

In a single-molecule transistor, a molecule is bridged across a 1- to 4-nm-wide electrode gap (see the first figure, middle panel). Three-terminal devices of this kind are powerful tools for exploring the fundamental physics of molecular devices: Parameters such as temperature, and electric and magnetic fields, may all be varied while the spectroscopic response is measured. Using single-molecule transistors, two groups (8, 9) have observed a unique type of quantum mechanical resonance, called a Kondo resonance, that can be correlated with a particular oxidation state—observed in solution-phase experiments—of the molecule [HN8].

Again, however, the devil is in the details. In particular, how the single-molecule transistors are made, the way in which molecules are assembled across the junctions, whether the molecules are bound covalently or noncovalently to the electrodes, and what electrode materials are used all play critical roles in either masking or revealing unique molecular electronic properties. Thus, despite early successes, it remains unclear whether single-molecule transistors can emerge as a general spectroscopic tool for guiding the development of molecular electronics.

A second active device is a two-terminal molecular switch tunnel junction [HN9] (see the first figure, bottom panel). The goal here is to design a molecule that, at a specific voltage, switches from a stable structure (isomer) to another, metastable isomer with a different conductivity and remains in the latter state until either another voltage pulse is applied or thermal fluctuations cause a return to the original isomer. The two states of the molecule correspond to the On and Off states of the switch, and the finite stability of the metastable state leads to a hysteretic current/voltage response that forms the basis of the switch. However, such switching behavior can also arise from the intrinsic device capacitance, from charge storage in defect sites at the molecule/electrode interface, or from electrochemical modification of the electrode materials (10). Such artifacts can be ruled out through careful control experiments, but some other, nonmolecular mechanism may nevertheless contribute to the switching response. Thus, the challenge is not just to rule out artifacts, but also to verify that the effect is molecular in origin by establishing a correlation to solution-phase observations.

We have previously reported (11) [HN10] on molecular switch tunnel junctions that contain a monolayer of bistable mechanically interlocked molecules—such as the [2]rotaxane (12) [HN11] shown in the lower panel of the first figure—sandwiched between silicon and metallic electrodes. These devices can be voltage-switched between a stable Off and a metastable On state. For the rotaxane case, we attributed these observations to an electrochemically driven translation (second figure, top panel) of the viologen-containing ring (colored blue) from the tetrathiafulvalene (green) site to the dioxynaphthalene (red) site to form the metastable state. The free energy barrier for relaxation back to the ground state provides an opportunity to correlate the device characteristics with molecular properties in solution.

A universal switch

(Top) Proposed electromechanical switching mechanism in bistable [2]rotaxanes. A positive voltage pulse oxidizes the ground state, resulting in the formation of the metastable state. During the oxidation, the ring (blue) moves from the green to the red site. The ground state is reformed thermally (rate-limiting step) or following a negative voltage pulse. A similar mechanism holds for bistable [2]catenanes (13). (Bottom) As shown in this Eyring plot, the kinetics of the rate-limiting step depend on the environment, reflecting a slowdown of the switching cycle as the free energy barrier increases from 16 to 21 kcal/mol. Environments: solution; self-assembled monolayer (SAM); polymer; molecular switch tunnel junction (MSTJ).

To establish this correlation, we performed variable temperature electrochemical measurements (13) to quantify the metastable-to-ground state relaxation of these molecular switches not only in solution, but also in self-assembled monolayers and in polymer matrices, as well as in the molecular switch tunnel junctions. The free energy barriers to relaxation (see the second figure, bottom panel) of the switches in these four different environments are, respectively, 16, 18, 18, and 21 kcal mol−1 at room temperature. Thus, although the corresponding relaxation rates significantly slow down by a factor of 10,000 as the molecules are increasingly confined, the mechanism remains the same: It is universal.

Several other groups have reported theoretical (14) and experimental (15) studies on similar molecular mechanical systems in various environments [HN12]. For example, Katz et al. have demonstrated a fuel cell in which a rotaxane self-assembled on gold electrode surfaces transports electrons from glucose oxidase to the electrode (15). The rotaxane bears a cyclophane that shuttles along the molecular string toward the electrode and back again within about 3 and 12 ms, respectively. Photo-driven relative movements (s) of the components in a hydrogen-bonded rotaxane have been demonstrated (16). Poleschak et al. have shown that mechanical movements in bistable, copper-based catenanes and rotaxanes display (17) lifetimes of microseconds to hours depending on their structures. The structures of molecular switches can thus govern switching kinetics (13). This discovery augurs well for achieving a fundamental goal in the field: chemical control over the physical properties of electronic devices.

Molecular electronics will mature into a powerful technology only if its development is based on sound scientific conclusions that have been tried and tested at every step. Reaching these objectives requires a detailed understanding of the molecule/electrode interface, as well as developing methods for manufacturing reliable devices and ensuring their robustness. Although applications involving single devices already exist (18) [HN13], the next-generation technologies will most likely consist of hybrid devices that combine molecular with existing electronics.

HyperNotes Related Resources on the World Wide Web

General Hypernotes

Dictionaries and Glossaries

E. Weisstein's World of Chemistry and World of Physics are made available by Wolfram Research.

A lexicon of semiconductor and related terms is provided by Intersil.

The TechEncyclopedia, a dictionary of electronics-related terms, is provided by TechWeb.

A nanotechnology glossary is provided by the Nanotechnology Now Web site.

Web Collections, References, and Resource Lists

The Google Directory has a section on nanotechnology.

The Yahoo Search Directory provides a section on nanotechnology resources.

The U.S. National Nanotechnology Initiative provides a collection of links and other resources.

NanoLink provides links to nanotechnology sites on the Web.

The Natelson Group, Department of Physics and Astronomy, Rice University, provides a collection of useful Internet links.

Online Texts and Lecture Notes

HyperPhysics is a tutorial on physics concepts presented by C. R. Nave, Department of Physics and Astronomy, Georgia State University.

Transistorized! is presented by PBS Online.

Molecular Electronics is a 1999 student project prepared for a course taught by S. Blanchard, Department of Biological and Agricultural Engineering, North Carolina State University.

Nanoword.net provides an introduction to nanotechnology and a glossary.

D. M. Gingrich, Department of Physics, University of Alberta, offers lecture notes on electronics.

P. Burke, Department of Electrical and Computer Engineering, University of California, Irvine, provides lecture slides in PDF format for a course on nanotechnology. A presentation on molecular electronics is included.

G.-Y. Wei, Division of Engineering and Applied Science, Harvard University, makes available lecture notes for a course on electronic devices and circuits.

Principles of Semiconductor Devices is a Web textbook provided by B. Van Zeghbroeck, Electrical and Computer Engineering Department, University of Colorado.

Y. Galperin, Department of Physics, University of Oslo, provides lecture notes in PDF format for a courses on modern solid state physics and quantum transport.

General Reports and Articles

The Heath Group at the California Institute of Technology makes available in PDF format a May 2003 Physics Today article by J. R. Heath and M. A. Ratner titled “Molecular electronics.”

The February 2002 issue of Materials Today had a review article by M. Ratner titled “Introducing molecular electronics” and a review article by K. Kwok and J. Ellenbogen titled “Moletronics: Future electronics.”

The 30 September 2002 issue of Chemical and Engineering News had an article by M. Jacoby titled “Nanoscale electronics.”

The presentations from a February 2004 workshop “Advances in Molecular Electronics: From molecular materials to single molecule devices” are made available by the Max-Planck-Institut für Physik komplexer Systeme.

The presentations from a July 2003 DURINT program review titled “The science and technology of nano/molecular electronics: Theory, simulation, and experimental characterization” are made available by the Applied Electronics Lab at Stevens Institute of Technology.

The October 2004 issue of Nanotechnology was a special issue on nanoscale devices.

Numbered Hypernotes

1. The field of molecular electronics. The Stoddard Research Group at the University of California, Los Angeles, offers introductions to molecular electronics and molecular machines. Molecular electronics was Science's 2001 Breakthrough of the Year. The 3 August 2001 issue of Science had a News Focus article on molecular electronics by R. Service titled “Assembling nanocircuits from the bottom up.” The 29 March 2002 issue had a News article by R. Service titled “Can chemists assemble a future for molecular electronics?” The 24 October 2003 issue had a New Focus article titled “Next-generation technology hits an early midlife crisis” (1) and the 21 November 2003 had a News of the Week article titled “Nanodevices make fresh strides toward reality,” both by R. Service; the 20 February 2004 issue had letters in response to the articles. Hewlet-Packard issued a 9 September 2002 press release titled “HP announces breakthroughs in molecular electronics”; more information is available on HP's Quantum Science Research Web page. The IBM Almaden Research Center offers a presentation on molecular electronics. Chemistry Highlights 2002 in Chemical & Engineering News includes links to articles on molecular electronics. The July 2000 issue of Wired had an article by R. Overton titled “Molecular electronics will change everything.”

2. Tunneling is defined in E. Weisstein's World of Physics. An introduction to quantum tunneling is provided by J. Schombert, Department of Physics, University of Oregon. P. Suranyi, Department of Physics, University of Cincinnati, provides lecture notes on tunneling for a course on modern physics.

3. Rectifier is defined in the TechEncyclopedia and in Intersil's lexicon. Information on rectification is provided by JC Physics. T. R. Kuphaldt's online textbook Lessons in Electric Circuits, Volume 3—Semiconductors has a chapter on diodes and rectifiers.

4. McConnell's prediction validated. The August 1961 issue of the Journal of Chemical Physics had an article by H. M. McConnell titled “Intramolecular charge transfer in aromatic free radicals” (2). Harden McConnell is in the Department of Chemistry, Stanford University. A profile of McConnell is provided by the Gallery of Chemists' Photo-Portraits and Mini-Biographies, Department of Chemistry, Michigan State University. The 1 September 1978 issue of the Journal of Chemical Physics had an article by E. E. Polymeropoulos and J. Sagiv titled “Electrical conduction through adsorbed monolayers” (4). The 24 June 2004 issue of the Journal of Physical Chemistry B had an article by T. Lee, M. A. Reed, et al. titled “Comparison of electronic transport characterization methods for alkanethiol self-assembled monolayers” (5). Takhee Lee is now at the Molecular Nanoelectronics Lab, Department of Materials Science and Engineering, Kwangju Institute of Science and Technology, Korea. The Mark A. Reed Research Group at Yale University provides presentations about their research on molecular electronics.

5. The 15 November 1974 issue of Chemical Physics Letters had an article by A. Aviram and M. A. Ratner titled “Molecular rectifiers” (3). The September 2003 issue of Chemical Reviews had an article by R. M. Metzger titled “Unimolecular electrical rectifiers” (6). The 9 June 2004 issue of the Journal of the American Chemical Society had an article by G. J. Ashwell et al. titled “Molecular rectification: Self-assembled monolayers in which donor-(π-bridge)-acceptor moieties are centrally located and symmetrically coupled to both gold electrodes” (7).

6. Alkanes. Alkane is defined in E. Weisstein's World of Chemistry. Entries on alkanes are included in Wikipedia and the Columbia Encyclopedia. The Chemistry for Physical Sciences Index, provided by Launceston College, Tasmania, Australia, offers a presentation on alkanes. An alkane tutorial is provided by Frostberg State University's Organic Chemistry Help.

7. Single-molecule transistors. The Ralph Group at Cornell University offers a research presentation on single-molecule transistors. The 28 March 1997 issue of Science had a Perspective by L. Kouwenhoven titled “Single-molecule transistors.” Small Times makes available an article by P. Brickley titled “Researchers report they have an atom surrounded on all sides” about research on single-molecule transistors. The October 2004 issue of Nanotechnology had an article by L. H. Yu and D. Natelson titled “Transport in single-molecule transistors: Kondo physics and negative differential resistance.”

8. Kondo resonance observed in single-molecule transistors. The 28 May 2004 issue of Science had a Perspective by N. S. Wingreen titled “Quantum many-body effects in a single-electron transistor” that discussed Kondo resonance. L. Kouwenhoven, Quantum Transport Group, Kavli Institute of Nanoscience, Delft, Netherlands, makes available in PDF format the January 2001 Physics World article by L. Kouwenhoven and L. Glazman titled “Revival of the Kondo effect.” D. Ralph, Laboratory of Atomic and Solid State Physics Cornell University, makes available in PDF format the 13 June 2002 Nature article by J. Park et al. titled “Coulomb blockade and the Kondo effect in single-atom transistors” (8). H. Park, Department of Chemistry and Chemical Biology, Harvard University, makes available in PDF format the 13 June 2002 Nature article by W. Liang et al. titled “Kondo resonance in a single-molecule transistor” (9) and the accompanying News & Views article by S. De Franceschi and L. Kouwenhoven titled “Electronics and the single atom.” The 17 June 2002 issue of Chemical & Engineering News had an article by M. Jacoby titled “Metal atoms take charge” about the research of the two groups.

9. Molecular switch tunnel junctions. J. O. Jeppesen, Department of Chemistry, University of Southern Denmark, makes available a poster presentation (in PDF format) that discusses molecular switches. The Heath Group makes available in PDF format a 2003 ChemPhysChem article by M. R. Diehl et al. titled “Single-walled carbon nanotube-based molecular switch tunnel junctions.”

10. Previous report of authors. The 18 August 2000 issue of Science had a Report by C. P. Collier et al. titled “A [2]catenane-based solid state electronically reconfigurable switch” (11).

11. Rotaxanes. An introduction to rotaxanes and catenanes is provided by the Leigh Group, School of Chemistry, University of Edinburgh; links to researchers are provided. A macromolecular nomenclature note by E. S. Wilks on the terminology and nomenclature for rotaxanes is available in PDF format from the Division of Polymer Chemistry, American Chemical Society. The 17 June 2002 issue of ChemPhysChem had an article by Y. Luo et al. titled “Two-dimensional molecular electronics circuits” (12); the Heath Group makes this article available in PDF format.

12. Other studies on molecular mechanical systems. The 6 October 2004 issue of the Journal of the American Chemical Society had an article by Y. H. Jang, S. Hwang, Y.-H. Kim S. S. Jang, and W. A. Goddard titled “Density functional theory studies of the [2]rotaxane component of the Stoddart-Heath molecular switch” (14). The 1 December 2004 issue of the Journal of the American Chemical Society had an article by E. Katz, O. Lioubashevsky, and I. Willner titled “Electromechanics of a redox-active rotaxane in a monolayer assembly on an electrode” (15). The 16 March 2001 issue of Science had a Report by A. M. Brouwer et al. titled “Photoinduction of fast, reversible translational motion in a hydrogen-bonded molecular shuttle” (16) and a related Perspective by J.-P. Sauvage titled “A light-driven linear motor at the molecular level.” An article by K. Patch about this research, titled “Light powers molecular piston,” appeared in Technology Research News. The February 2004 issue of Chemical Communications had an article by I. Poleschak, J.-M. Kern, and J.-P. Sauvage titled “A copper-complexed rotaxane in motion: Pirouetting of the ring on the millisecond timescale” (17). A research presentation titled “Molecular machines based on copper complexes” is provided by J.-P. Sauvage's Laboratoire de Chimie Organo-Minérale at the Université Louis Pasteur, Strasbourg, France.

13. An existing application. The Summer 2000 issue of Diabetes Technology & Therapeutics had an article by B. Feldman et al. titled “FreeStyle: A small-volume electrochemical glucose sensor for home blood glucose testing” (18). The TheraSense Web site provides information about the FreeStyle system and the technology involved.

14. Amar H. Flood and J. Fraser Stoddart are in the Department of Chemistry and Biochemistry, University of California, Los Angeles, and at the California NanoSystems Institute.

15. David W. Steuerman is in the Department of Physics, University of California, Santa Barbara.

16. James R. Heath is in the Division of Chemistry and Chemical Engineering, California Institute of Technology.

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