Molecular Scale Electronics
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circuitry. Our strategy for demonstrating these ideas has involved devising synthetically precise routes to conjugated rigid rod molecules, probing their electrical properties, and devising a method of self-assembly for these molecules which will allow for the fabrication of molecular electronic devices. While it is well known that bulk conjugated materials can be made semiconducting or even conducting when doped [11], only recently have we discovered how thiol-ended rigid rod conjugated molecules orient themselves on gold surfaces in self-assembled monolayers (SAMs), and how we could record electronic conduction through these undoped conjugated molecules [10]. Furthermore, we have made significant advances in the synthesizing molecules to serve as molecular electronic devices. We have also demonstrated the attachment of protected thiol moieties to one or both ends of the molecular wires to serve as molecular-scale alligator clips that provide electrical contact to gold probes [12]. Using this technology, we have demonstrated negative differential resistance (NDR) behavior in a SAM of thiol-ended rigid rod conjugated molecules [13]. This result demonstrates that these molecules can be used as nanoscale electronic devices and brings the molecular computer concept much closer to being a reality. RESULTS AND DISCUSSION Synthesis of Molecules Which Can Serve as Molecular Scale Devices As mentioned above, discrete, highly conjugated oligo(phenylene ethynylene)s as molecular scale wires have been demonstrated to conduct an electric current. Furthermore, STM studies demonstrate that alkyl units pose significantly larger electronic transport barriers than the ð-conjugated moieties in single molecule systems [14]. With this knowledge in hand, we then began designing molecules which would, at least in theory, serve as molecular electronic devices. Molecules 1 and 2 are two examples of the kinds of ð-conjugated molecules we have synthesized. Furthermore, functional groups can be added to the rings to drastically alter their solubility and electronic properties. We can introduce methylene and ethylene barriers into the conductive wires or devices to impart a conduction barrier as shown in molecules 3 and 4. Such barriers may SAc
AcS
SAc
1
H5 C2
C2H 5
C 2H 5
2 SAc
AcS 28 Å
allow for the development of molecular devices which might serve as field effect transistors or resonant tunneling diodes. While three-terminal systems may possess
switch-like properties, four-terminal systems (like molecule 5) might serve as logic gates [15].
AcS
C H2
CH2
SAc
3
27 Å
29 Å AcS
SAc
C 2H 5
H 5C 2
CH 2
30 Å
C2 H5
SAc
4
Self-Assembled Monolayer Formation of Rigid-Rod Conjugated Oligomers As a prelude to the utilization of molecular scale wires in molecular scale electronic devices, it is necessary to understand the molecular ordering on metal surfaces. These sulfur-terminated conjugated oligomers form SAMs on gold surfaces by attachement of the thiol end groups which serve as molecular scale alligator clips [12], this i
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