The Stark Effect: A Tool for the Design of High-Performance Molecular Rectifiers.

Molecular electronic devices offer a path to the miniaturization of electronic circuits and could potentially facilitate novel functionalities that can be embedded into the molecular structure. Given their nanoscale dimensions, device properties are strongly influenced by quantum effects, yet many of these phenomena have been largely overlooked. We investigated the mechanism responsible for current rectification in molecular diodes and found that efficient rectification is achieved by enhancing the Stark effect strength and enabling a large number of molecules to participate in transport. These findings provided insights into the operation of molecular rectifiers and guided the development of high-performance devices via the design of molecules containing polarizable aromatic rings. Our results are consistent for different molecular structures and are expected to have broad applicability to all molecular devices by answering key questions related to charge transport mechanisms in such systems.

Humidity sensors based on molecular rectifiers.

Ambient humidity plays a key role in the health and well-being of us and our surroundings, making it necessary to carefully monitor and control it. To achieve this goal, several types of instruments based on various materials and operating principles have been developed. Reducing the production costs for such systems without affecting their sensitivity and reliability would allow for broader use and greater efficiency. Organic materials are prime candidates for incorporation in humidity sensors given their extraordinary chemical diversity, low cost, and ease of processing. Here, we designed, assembled and tested humidity sensors based on molecular rectifiers that can electrically transduce the changes in the ambient humidity to offer accurate quantitative information in the range of 0 to 70% relative humidity. Their operation relies on the changes occurring in the electric field experienced by the molecular layer upon absorption of the polar water molecules, resulting in modifications in the height and shape of the tunneling barrier. The response is reversible and reproducible upon multiple cycles and, coupled with the simplicity of the device architecture and manufacturing, makes these nanoscale sensors attractive for incorporation in various applications.

Intermolecular charge transfer enhances the performance of molecular rectifiers.

Molecular-scale diodes made from self-assembled monolayers (SAMs) could complement silicon-based technologies with smaller, cheaper, and more versatile devices. However, advancement of this emerging technology is limited by insufficient electronic performance exhibited by the molecular current rectifiers. We overcome this barrier by exploiting the charge-transfer state that results from co-assembling SAMs of molecules with strong electron donor and acceptor termini. We obtain a substantial enhancement in current rectification, which correlates with the degree of charge transfer, as confirmed by several complementary techniques. These findings provide a previously enexplored method for manipulating the properties of molecular electronic devices by exploiting donor/acceptor interactions. They also serve as a model test platform for the study of doping mechanisms in organic systems. Our devices have the potential for fast widespread adoption due to their low-cost processing and self-assembly onto silicon substrates, which could allow seamless integration with current technologies.