Overview of Nanoelectronic Devices <s> D. Quantum-Effect Molecular Electronic Devices <s> Nanometer-size Au clusters deposited from a cluster beam onto a thin dithiol film were studied at room temperature using an ultrahigh vacuum scanning tunneling microscope (STM). The dithiol molecules tether the deposited Au clusters to the underlying gold substrate and repeatable STM scans of the Au clusters were achieved. Data of the tunneling current as a function of applied voltage yield reproducible evidence for single electron tunneling at room temperature. By fitting the measured I(V) data to a Coulomb blockade model, estimates for the electrical resistance of a single dithiol molecule are also obtained. <s> BIB001 </s> Overview of Nanoelectronic Devices <s> D. Quantum-Effect Molecular Electronic Devices <s> Double-ended aryl dithiols [alpha,alpha'-xylyldithiol (XYL) and 4,4'-biphenyldithiol] formed self-assembled monolayers (SAMs) on gold(111) substrates and were used to tether nanometer-sized gold clusters deposited from a cluster beam. An ultrahigh-vacuum scanning tunneling microscope was used to image these nanostructures and to measure their current-voltage characteristics as a function of the separation between the probe tip and the metal cluster. At room temperature, when the tip was positioned over a cluster bonded to the XYL SAM, the current-voltage data showed "Coulomb staircase" behavior. These data are in good agreement with semiclassical predictions for correlated single-electron tunneling and permit estimation of the electrical resistance of a single XYL molecule (approximately18 ± 12 megohms). <s> BIB002
Using molecular structures for quantum confinement might make it possible to manufacture fast, quantumeffect switching devices on a large scale more uniformly and cheaply than has thus far been feasible with solid semiconductors. Early on, investigators in molecular electronics proposed incorporating into molecules and supramolecular structures potential wells for the quantum confinement of mobile electrons  , and several groups now are working on this approach for molecular switching  ,  . The goal has been to implement the electricfield controlled resonant tunneling and single-electron switching effects that have been discussed above for solids. These efforts include embedding in supramolecular structures metal nanoclusters that exhibit properties like QD's, such as Coulomb blockade. This is the approach adopted in the aforementioned work at Purdue, which showed that large numbers of such QD's could be manufactured with great uniformity, then self-assembled into an extremely regular structure BIB002 , BIB001 . Alternatively, a quantum well might be embedded in a molecular wire like that in Fig. 7 by inserting pairs of barrier groups that break the sequence of conjugatedorbitals discussed above. This has been proposed by Tour  , who suggests the structure shown in Fig. 8 for a wire with such barriers inserted. That structure would form a two-terminal molecular RTD, but structures for threeterminal RTT-like molecules also have been suggested  , [169] . A difficulty in realizing such molecular RTD's and RTT's is the fact that the charging energy in such a small potential well is sure to be large, perhaps larger than the energy spacing between the levels in the well. This could make the device much more like a QD than a solid-state RTD. Also, unlike a solid-state RTD, such as is diagramed in Fig. 3 , there is not a near continuum of unoccupied energy levels on the drain side of the molecule to allow an electron in the quantum well to tunnel out, so achieving resonance with the source and drain simultaneously may be difficult. Still, the flexibility and variety of such organic structures gives the designer many variables with which to optimize  . (a) Conducting chain molecule, like that shown in Fig. 7 , but with insulating barrier groups that may generate (b) potential well for quantum confinement that could (c) create a resonant tunneling effect when the molecule is subjected to a voltage bias, permitting a current of electrons to be transmitted through the device. Compare this schematic with that in Fig. 3 for an analogous, but much larger, solid-state device. device performance. The recent demonstration of electromechanically controlled resonant tunneling in a molecule also is very encouraging. These facts and the measurements of Reed and Tour  on structures like that in Fig. 7 give the authors a sense of optimism that an efficient fieldcontrolled molecular quantum-effect switching device can be engineered successfully in the foreseeable future.