Switches lie at the heart of electronics and their design puts a limit on the size of integrated circuits. By harnessing chemistry, researchers have reduced this problem to a molecular level.
In the drive towards ever smaller electronic devices, many nanotechnologists are trying to exert chemical control over their components. Integrating molecular functions into this technology could lead to new types of electronic device. On page 67 of this issue Schiffrin and co-workers1 take a first step towards synthesizing reversible molecular switches, and integrating these switches into a simple electronic circuit.
Take apart any digital microelectronic device and you will find basic electronic switches, known as transistors and diodes. More complex circuit elements, such as flip-flops, inverters or Schmitt triggers, are created from various permutations of these building blocks. For example, when two transistors are combined to form a Schmitt trigger, they create a circuit that changes from ON to OFF only if a voltage input exceeds a specific value. In this way, a Schmitt trigger defines a single bit of binary information (0 or 1). Schmitt triggers and flip-flop circuits are in turn combined to form more complex integrated circuits.
In their system, Schiffrin and co-workers switch their molecule's conductivity on and off by changing its oxidation state. The number of electrons associated with an atom determines its oxidation state. Some organic molecules contain a 'redox' centre where reduction (electrons added) or oxidation (electrons removed) occurs both readily and reversibly. If they are sandwiched as molecular layers between electrical contacts, such organic molecules can support relatively large currents2. They do this through 'resonant tunnelling', a phenomenon that occurs when the electron energy levels of the molecule overlap with those of the metal it is in contact with. This overlap is more likely to occur in molecules that are easily oxidized or reduced.
Schiffrin and co-workers1 now show that a similar process can occur in a much smaller system. The authors have shown previously3 that a layer of an organic molecule containing the bipyridinium group (bipy) sandwiched between a gold electrode and gold nanoparticles can easily gain an electron and be reduced from the cation bipy2+ to the radical ion bipy●+. In their latest experiment, they attached an alkanethiol 'clip' to each end of a bipy molecule. One thiol was connected to the gold electrode and the other made contact with a gold nanocluster 6 nanometres in diameter (Fig. 1). The authors used the tip of a scanning tunnelling microscope to record the electrical properties of the nanoparticles individually. Their results show clearly how changing the redox state of the bipy molecules can control electron transport between the gold contacts. In other words, the molecules behave much like a molecular switch. The authors estimate that their 'molecular device' corresponds to no more than 60 organic molecules and needs fewer than 30 electrons to operate.
The way in which this new molecular device works is similar to the basic function of a Schmitt trigger. When the molecule is in the reduced bipy●+ state, relatively large currents flow through the nanocluster–molecule– electrode set-up characteristic of resonant tunnelling (Fig. 1). But when a certain threshold voltage is applied to the gold electrode, the tunnelling current decreases markedly. The threshold voltage corresponds to the oxidation of bipy●+ to bipy2+.
Schiffrin and co-workers have created an electrochemical switch whose state is determined by the potential needed to reduce the molecule by one electron. Importantly, the threshold potential for ON/OFF switching should be tunable over a wide range simply by choosing molecules with easily reversible yet varying redox states. But will redox-switchable integrated nanoelectronic circuits ever be made? The answer is almost certainly no. A 'molecular' Schmitt trigger would operate too slowly and have insufficient gain (signal amplification) to be useful.
Nonetheless, there are possible applications for redox-switchable nanoelectronic devices where gain is not important2,4. For example, chemical sensors based on the type of ligand-modified gold nanocluster described by Schiffrin and co-workers can, in theory, detect single molecules or single chemical reactions4. In addition, new forms of computing logic and memory are also being designed around redox-switchable molecular architectures4,5.
The system reported by Schiffrin and colleagues adds to a rapidly expanding list of chemically or electrochemically switchable nanoelectronic devices. Earlier this year, Heath and co-workers5 showed that the resistance of a monolayer of catenanes can be reversibly altered by several orders of magnitude using an electrochemically induced switching mechanism. Collins and co- workers6 showed that the electrical properties of carbon nanotubes can be switched chemically by dousing them with certain gases. Before these studies, work by my group had shown that single-electron tunnelling energies in individual gold nanoclusters can be controlled by adding protons to ligands bound to the cluster surface7.
Perhaps the most significant outcome of these studies will be a deeper fundamental understanding of basic relationships between structure and electronic function in nanoscale structures. Future electronic devices may not contain flip-flops, or any other circuits we know now. By exploring the electrical properties of integrated molecular nanoelectronics, new electronic properties and applications are certain to emerge in the coming years.
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