The control of charge transport in an active electronic device depends intimately on the modulation of the internal charge density by an external node. For example, a field-effect transistor relies on the gated electrostatic modulation of the channel charge produced by changing the relative position of the conduction and valence bands with respect to the electrodes. In molecular-scale devices, a longstanding challenge has been to create a true three-terminal device that operates in this manner (that is, by modifying orbital energy). Here we report the observation of such a solid-state molecular device, in which transport current is directly modulated by an external gate voltage. Resonance-enhanced coupling to the nearest molecular orbital is revealed by electron tunnelling spectroscopy, demonstrating direct molecular orbital gating in an electronic device. Our findings demonstrate that true molecular transistors can be created, and so enhance the prospects for molecularly engineered electronic devices.

Single molecule electronic devices in which individual molecules are utilized as active electronic components constitute a promising approach for the ultimate miniaturization and integration of electronic devices in nanotechnology through the bottom-up strategy. Thus, the ability to understand, control, and exploit charge transport at the level of single molecules has become a long-standing desire of scientists and engineers from different disciplines for various potential device applications. Indeed, a study on charge transport through single molecules attached to metallic electrodes is a very challenging task, but rapid advances have been made in recent years. This review article focuses on experimental aspects of electronic devices made with single molecules, with a primary focus on the characterization and manipulation of charge transport in this regime.

A high-potential porphyrin is codeposited on TiO 2 nanoparticles together with our Cp*-iridium water-oxidation catalyst to give a photoanode for a water-splitting cell. The photoanode optically resembles the porphyrin yet electrochemically responds like the Ir catalyst when it is immersed in aqueous solutions. Photoelectrochemical data show that illumination of the codeposited anode in water results in a marked enhancement and stability of the photocurrent, providing evidence for light-induced activation of the catalyst.Chart 1 Compounds 1 and 2.

The changes of molecular conformation, contact geometry, and metal-molecule bonding are revealed by inelastic-electron-tunneling spectroscopy measurements characterizing the molecular vibrational modes and the metal-phonon modes in alkanedithiol molecular junctions at low temperature. Combining inelastic-electron-tunneling spectroscopy with mechanical control and electrode material variation (Au or Pt) enables separating the influence of contact geometry and of molecular conformation. The mechanical strain of different electrode materials can be imposed onto the molecule, opening a new route for controlling the charge transport through individual molecules.

Intermolecular chain-to-chain tunneling in metal−alkanethiol−metal junctions was investigated by measuring the molecular-tilt dependence of the tunneling through a molecular ensemble of alkanethiols using conducting atomic force microscopy. A variable tip-loading force was applied to tilt the molecular configuration while measuring the current−voltage characteristics of the molecular junctions. The observed transport through the molecules exhibited a tilt angle dependent intermolecular charge transfer in addition to the tunneling along the molecular chains.