Future and Emerging Technologies- Microelectronics Advanced Research Initiative

ftp://ftp.cordis.europa.eu/pub/esprit/docs/melnarm.pdf

2.2.4.2 Electric-field controlled molecular electronic switching devices

Among molecular switches, electric-field controlled molecular electronic switching devices are closest to conventional semiconductor devices and therefore the most likely candidates for applications. The basic idea is to use the central part of molecules for functions (switching, RTD,
transistor, rectification, etc….) while the endgroups are used to self-assemble. Diblock copolymers, for example, may be self-assembled into chains and purified to very high yields. Thiol endgroups can be used to anchor the copolymers to metal electrodes.

The mechanism for the functional behaviour can be very diverse. As already indicated, attaching electrophile and electrophobic groups to either end of the molecule induces rectifying behaviour. Aviram [Aviram 1988] proposed a number of molecules that switch between conducting and insulating states by oxidation / reduction. One example is tetrathiafulvalium (+) with a partially filled highest occupied molecular orbital to allow free states for conduction and tetrathisfulvalene which is an insulator because of its filled highest molecular orbital. The transport properties are very similar to Coulomb blockade (Figure 2.4). Current voltage data from single designer molecules have been demonstrated such as the rectification properties of a benzene ring attached to two gold electrodes using S atoms (Benzene-1, 4-dithiolate) [Reed 1997]. More complicated structures have also been demonstrated where the rotation of a single bond of the p orbitals allows the conductivity to be changed by over 6 orders of magnitude, providing a switching mechanism in a polymer chain [Reed 1998].
The first “molecular transistor” actions were shown by R.P. Andes [Andes 1996] using selfassembled gold particles on top of a,a’-xyldithiol (XYL) molecules on an Au(111) surface. The
second electrode was a STM tip and a Coulomb staircase function was demonstrated at room temperature.

A second demonstration was using carbon nanotubes placed across two electrodes with a controlling back-gate [Tans 1998]. Clear Coulomb blockade and also single electron transistor
properties were demonstrated at room temperature. In semiconducting nanotubes, “classical” field effect transistor action could be obtained [Tans 1998, IBM]

These experiments demonstrate the basic transistor principles but neither technique is suitable
for mass manufacture. A number of proposals have appeared in the literature for molecular devices but few have been demonstrated. No real molecular three terminal transistor device defined in this roadmap has yet been demonstrated. Significant progress, however, which has been demonstrated in the literature is discussed below.
Recognition of conformation and adaptation of individual molecules has been demonstrated
where the STM is used to understand bond rotations in molecules and to determine how they adapt or can switch their state. Such conformational processes are used in natural systems and form a basis for molecular switches [Jung 1997]. It has been demonstrated that molecular systems such as fullerene based systems can be incorporated in two and three terminal devices and that interesting electronic characteristics can be obtained, i.e. macroscopic quantum state in a bucky tube [Tans 1997].

The area of self-assembly has for some time been subject to designer molecular concepts

through optimisation of molecular chain length and omega functionalisation as well as techniques
such as contact printing to define molecular patterns on the scale of tens of nanometers. Lehn’s
[Lehn 1995] supramolecular concepts have also been explored in the design and self-organisation of grid-like metal co-ordination arrays using molecular systems [Schubert 1997].
The assembly of devices and arrays may be expected to be radically different from current approaches. For instance, at the macroscopic scale chemical interactions can be used for the three dimensional self-assembly of millimetre-scale components [Terfort 1997]. A different form of molecular recognition is to use DNA to create a binding interaction between nanocrystals of colloidal metals in solution (or potentially at a surface) [Mirkin 1996].

2.2.4.3 Alternative Approaches to Molecular Switching

Electromechanical Switching Several illustrations of STM based molecular manipulation have in recent years been proposed as a means for performing electronic computation. Although the need of UHV and the intrinsically low speed of the STM precludes any applications, we briefly mention certain examples of these thought-provoking experiments.
The main consideration of electromechanical switching at the molecular level is that the properties are controlled by deforming or reorientating a molecule rather than moving the electrons on the molecule. An example is the C60 based single molecule electromechanical amplifier, which has been demonstrated by deforming the molecule using a STM tip. The device relies on the controlled vertical deformation of the C60 cage resulting in mechanical modification of resonance tunnelling bands. This demonstrates a fundamentally new approach to switching and amplification using molecular mechanics and quantum processes. The speed of such a device would be limited by the vibrational frequency of C60 at over 10 THz (1013 Hz) although much lower frequencies (10 Hz) have been demonstrated [Joachim 1997].


Another example is the Molecular Abacus device using the fullerene C60 as beads, a 0.25 nm high monatomic step as the rod and STM tip as the finger to both reposition in one dimension and count by imaging. The abacus itself has an active area defined as around 1 nm x 13 nm and is a molecular mechanical device. [Cuberes 1996]. By moving a single atom out or into an atom wireresearchers at Hitachi have simulated a two state electronic device of atomic dimensions [Wada 1993]. The device is called an atom relay or molecular relay. Two-state devices based on the change of a molecular conformation can also make a relay such as a rotamer [Takeda 1982].
Photoactive / photochromatic switching A number of molecules, specifically a number of proteins, may have their electron distribution changed by the absorption of photons to produce switching effects. The biological photochrome bacteriorhodopsin has been suggested as one possible type of molecule for holography, spatial light modulators, neural network optical computing, nonlinear optical devices, and optical memories [Birge 1995]. The significance of bacteriorhodopsin stems from its biological function as a photosynthetic proton pump in the bacterium Halobacterium halobium. A combination of serendipity and natural selection has yielded a native protein ideal for optoelectronic applications. Other examples of optoelectronic biological molecules include visual rhodsin [Birge 1995], chloroplasts [Greenbaum 1992] and photosynthetic reaction centres [Boxer 1992].

2.2.4.4 Molecular Wires

A further important problem, which has escaped much attention, is that most polymers are
very poor conductors and hence molecules must be found with good conducting properties if circuits are to be built up. Note that utilising the delocalised electrons on aromatic and acetylene
groups is not sufficient to obtain good conducting properties as required by the microelectronics
industry in circuits. The delocalised p electronics in such compounds occupy the “homo” (highest
occupied molecular orbital) fully, so that for charge transport an extra electron must be added to the “lumo” (lowest unoccupied molecular orbital), which is typically a few eV away. Thus, the compound acts as a semiconductor and good conducting properties can only be obtained by doping the systems. The only metallic wire like molecules that have been studied in more detail are carbon nanowires, where the bandstructure allows metallic behaviour. These compounds seem quite promising for some applications, but ways must be found to connect them, through conducting junctions at specified positions, to the active switching element. The RNA synthesis pursued by the Technion group could be a useful approach in this direction.