With the help of a gate electrode to control the charge state of individual molecules on graphene, information can be moved along a one-dimensional molecular chain, mimicking the behaviour of an electronic shift register.
Individual molecules and molecular nanostructures could be used to build future nanoelectronic devices1,2,3,4. Organic molecules are attractive for such purposes because they are cheap, consist of abundant elements, and their valuable features can be optimized via the sophisticated techniques of modern organic synthesis. Their charge transport processes can also be controlled by external stimuli through conformational, chemical and electronic modifications of the molecules, which is key to adding active functionalities to molecular nanodevices. However, in order to develop practical devices several challenging obstacles need to be overcome, including the lack of scalable nanofabrication methods, limited device and operation stabilities, and insufficient control over the state (electronic, conformational or chemical) of the molecular nanostructures. Writing in Nature Electronics, Michael Crommie and colleagues now show that the charge state of molecular arrays on graphene can be precisely controlled by the application of a gate voltage, creating collective charge patterns that can made to function as a charge-based shift register5.
The approach hinges on previous work from the same group in which a gate electrode was added to a scanning tunnelling microscope (STM) in order to allow gate-controlled tuning6. Conventional STMs can be thought of as two-terminal devices — tip and sample — that allow rectifying but not gating behaviour7. However, by inserting a graphene/boron nitride electrode between the tip and the sample (which itself is insulated by a surface layer of silicon dioxide) the gating of molecular nanostructures deposited on the graphene surface is possible. By injecting a tunnelling current between the STM tip and the graphene electrode, while also applying gate voltages between the silicon sample and the graphene, the different charge states of the graphene-supported molecules can be studied. Because the charge state decisively affects the physical and chemical properties of the molecules8, achieving such external control is essential for fundamental studies9 and the establishment of device applications6. In this latest development, the researchers — who are based at institutions in the US, China, UK, Germany, Japan and Singapore — report the first device-like architecture that exploits this gating approach in order to control the charge states of individual molecules.
To achieve this, one-dimensional molecular arrays are first created via molecular self-assembly9. In particular, an organic template is fabricated by molecular self-assembly of 10,12-pentacosadiynoic acid (PCDA) molecules, which form elongated islands on the graphene with almost defect-free edges. Next, fluorinated tetracyanoquinodimethane (F4TCNQ), which is a strong electron acceptor, is added and attaches only to the edges of the moiré pattern of the PCDA molecules, creating molecular arrays with the same periodicity (1.92 nm) as the moiré lattice. This distance is interesting because the molecules are far enough apart to exclude orbital hybridization among themselves, but still close enough to experience Coulomb repulsion if the molecules are in a charged state. With the setup, the charge states of each individual molecule can be distinguished due to different electronic and vibronic features in the I–V spectra recorded by sweeping the voltage between the graphene electrode and the STM tip, and recording the tunnel current intensity for different values of the gate voltage.
For negligible gate voltages, all the molecules show the features of the neutral species, as might be expected, whereas for high gate voltages all become negatively charged. However, in the intermediate range of gate voltages a new phase with alternating charged and neutral molecules along the chain appears (Fig. 1), which the researchers attribute to the Coulomb repulsion that would arise if two charged molecules were to occur in neighbouring sites. They also predict an interesting effect at the terminal molecules of finite chains: the molecules at these sites should prefer to always be in a charged state, due to the low repulsion associated with the lack of neighbours on the empty side of the finite chain.
Both of these effects are employed in the experiment where the molecular array functions as a shift register. A molecule is removed from one end of the chain by tip manipulations. The next molecule in the chain, which was neutral before the manipulation, now becomes charged since it is a terminal molecule. To maintain the alternating pattern between charged and neutral molecules, the entire pattern then shifts along by one molecule. If the charge states of the molecules are considered as information bits sitting at specific molecular sites, the whole pattern of bits is shifted by one position, mimicking an electronic shift register.
Exploiting this effect to fabricate practical operating devices will, of course, require finding an alternative method to actuate it than mechanical STM manipulations, since those are hard to automate. Furthermore, the thermal stability of the molecular design will need to be improved, since the non-bonding interactions involved in the stabilization of supramolecular structures are too weak to be stable at room temperature. Still, the work by Crommie and colleagues provides a striking demonstration of the potential of the gating approach to develop new concepts in molecular electronics by allowing an unprecedented tunability of the molecular charge state. In particular, the transmission of information along relatively long molecular chains using these collective charge states allows us to consider new methods for transmitting and acting on information bits in molecular nanoelectronics.
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The author declares no competing interests.
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Otero, R. A shift in molecular devices. Nat Electron 3, 584–585 (2020). https://doi.org/10.1038/s41928-020-00490-9