Transistors have been made from single molecules, where the flow of electrons is controlled by modulating the energy of the molecular orbitals. Insight from such systems could aid the development of future electronic devices.
Transistors, the fundamental elements of integrated circuits, control the flow of current between two electrodes (the source and drain electrodes) by modifying the voltage applied at a third electrode (the gate electrode). As manufacturers compete to produce ever smaller devices, one logical limit to circuit miniaturization is transistors whose channels are defined by a single molecule. The construction and characterization of such a device has been a long-standing goal of nanoelectronics.
In fact, solid-state molecular transistors have already been made, and are based on two mechanisms: Coulomb blockade, in which the flow of electrons is controlled by the sequential charging of a molecule; and the Kondo effect, in which conducting electrons interact with local spin (intrinsic angular momentum) in a molecular junction1,2,3,4. A third approach has been predicted5, based purely on electrostatic modulation of the molecular-orbital energy of a single molecule. On page 1039 of this issue, Song et al.6 describe the first practical realization of this approach, and characterize the resulting devices in unprecedented detail.
Despite various reports1,2,3,4,7 of single molecules being electrically connected to two electrodes, achieving this feat is a daunting task. Song et al.6 fabricated their devices by coating gold wires with the molecules of interest, then breaking the wires using a technique called electromigration and thus producing nanometre-scale gaps in the wires (Fig. 1). On occasion, the molecules coating the wire fortuitously become trapped in the gaps. This results in systems of source/drain electrodes — two broken ends of the wire — spanned by the molecules, forming a junction through which electrons can 'tunnel'. One of the benefits of this fabrication method is that the junction can be formed directly over an oxidized-aluminium gate electrode, thus providing the necessary three-terminal geometry of a transistor.
Where Song and colleagues' study excels is in the detailed examination of their molecular junctions. The field of molecular electronics has long been plagued by concerns that the observed current–voltage characteristics of reported devices are caused by impurities or defects in the systems, rather than by the molecular species under study. Song et al.6 have avoided such uncertainties by thoroughly characterizing the charge-transport properties of their devices using a combination of spectroscopy techniques in situ. In this way, they provide unprecedented insight into the underlying physics of charge transport in their molecular transistors.
The authors used inelastic electron tunnelling (IET) spectroscopy to measure the interactions between the tunnelling electrons and the vibrational modes of the molecules in their devices. This technique provides definitive proof that the measured currents actually pass through the molecules in single-molecule transistors, and yields some information about the pathways taken by electrons as they cross the junctions8. The authors tested two types of transistor, each with a different molecule in the junction — either an alkane dithiol (which contains two SH groups connected by a saturated hydrocarbon chain) or an aromatic dithiol (which contains two SH groups connected by a benzene ring). Because each dithiol has its own vibrational 'fingerprint', the IET spectra of the devices provide unambiguous evidence of the molecules in the junctions.
The second technique used by Song et al. was transition-voltage spectroscopy. Electrons crossing a molecular junction do so using one of two tunnelling mechanisms that depend on the magnitude of the source–drain voltage; the transition voltage (Vtrans) is the voltage at which tunnelling switches from one mechanism to the other. It has previously been shown9 that Vtrans is proportional to the difference in energy between the gating orbital of the molecular junction (the orbital that modulates electron tunnelling) and the Fermi levels of the source and drain electrodes, where the Fermi level is the highest possible energy for a conducting electron in an electrode. By measuring Vtrans using transition-voltage spectroscopy at different applied gate voltages, Song et al. demonstrated that a linear relationship exists between gate voltage and molecular-orbital energy in their devices, as expected for single-molecule transistors.
The nature of the molecular orbital that couples to the tunnelling electrons (that is, whether or not the orbital is occupied by electrons from the molecule in the junction) can be determined from the change in conductance of the transistor with respect to the applied gate voltage. Song et al. found that both of their transistor types become more conducting when a negative gate voltage is applied. Because negative gate voltages lower the energy difference between the highest occupied molecular orbital (HOMO) of the molecular junction and the electrode's Fermi level, this indicates that, in their device, tunnelling electrons couple to the HOMOs of the molecules.
The authors found further evidence that the current through their device was gated by the HOMO energy of the molecular junction by examining the dependence of the IET spectra on the applied gate voltage. The IET spectra of the transistors that incorporate alkane dithiols were essentially unaffected by the gate voltage. This indicates that electron tunnelling through the device is always 'non-resonant', that is, there is a large energy difference between the dithiol's HOMO and the electrode's Fermi level.
Conversely, Song et al. observed that the applied gate voltage strongly modulates the IET spectra of transistors that incorporate an aromatic dithiol. Specifically, when a negative gate voltage is applied (which brings the energy of the molecular junction's HOMO closer to that of the electrode's Fermi level), the signal intensities of the spectra increase greatly and the shapes of the vibrational peaks change. The change in peak shape is a clear indication of increased coupling between the tunnelling charge carrier and the molecular vibrations, owing to a resonance between the HOMO and the Fermi level10. The authors have thus provided the first experimental demonstration that resonant and non-resonant vibrational coupling can be tuned in single-molecule transistors.
One of the most surprising features of Song and co-workers' study6 is the strong effect of the gate voltage on the molecular-orbital energy of their device. For both dithiols studied, the molecular orbitals shifted in energy by 0.25 electronvolts when 1 volt was applied to the gate electrode, a remarkably strong coupling. It is unclear why the gate coupling should be so strong, but the most likely explanation is that the molecules are extremely close to, or in intimate contact with, the gate dielectric (the oxidized aluminium of the gate electrode). The need for such precise alignment may in part explain why so few of the devices prepared by the authors functioned properly as transistors — only 35 out of 418 were found to have the desired current–voltage characteristics.
Through their multi-spectroscopy approach, Song et. al.6 have provided the first conclusive evidence that a solid-state molecular transistor can function through the relative alignment of its orbital energies with the electrode's Fermi level, and that this alignment can be efficiently tuned by the applied gate voltage. Their work sets a benchmark for the validation of future studies of charge transport in molecular systems. But much work remains to be done before molecular electronic devices can effectively compete with their larger silicon-based brethren. For example, a fabrication method that provides high yields of densely packed single-molecule devices has yet to be developed. In the meantime, Song and colleagues' work provides an excellent foundation for further development of well-characterized molecular devices.
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