Getting a Charge Out of Nanotechnology
Ever since the dawn of the electronic age, science has been on a quest for miniaturization, making transistors smaller in order to integrate many into a circuit and further miniaturizing to build microprocessors. The potential for further miniaturization is predicated on physics: The tiniest transistors must still be large enough to control the on or off flow of electrons. The smallest transistors that can be envisioned consist of just a few atoms or a small molecule. The potential for making such small electronic circuits has captivated Latha Venkataraman, associate professor of applied physics.
“By developing circuits with components that consist of a single molecule (a collection of a few atoms), I am working at a size regime that is close to the fundamental limit,” she says. “There’s something exciting about exploring how things work at such an extreme and largely uncharted scale.”
The underlying focus of Venkataraman’s research is to fabricate single-molecule circuits, a molecule attached to two electrodes, with varied functionality where the circuit structure is defined with atomic precision. That’s the holy grail of molecular electronics and a goal Venkataraman had been pursuing even when popular science relegated the idea to theory.
“When I started at Columbia, the field of molecular electronics was plagued with uncertainty. No one could imagine that a single-molecule circuit could actually function reliably and reproducibly,” she explains. Through collaboration with chemistry groups at Columbia, a method was devised to create workable single-molecule devices.
“This was a major breakthrough, because we could finally probe the properties of such circuits and relate these to the chemical and physical natures of the molecules,” she says.
Now, Venkataraman is focused on learning how these circuits work and how to optimize their function as active device components. Her findings will also enhance the understanding of charge transport in molecular systems and across metal-organic interfaces, with impact on the fields of organic electronics, photovoltaics, catalysis, and even biological processes (such as photosynthesis and respiration).
“We are now measuring how electronic conduction and single bond-breaking forces in these devices relate not only to the molecular structure but also to the metal contacts and linking bonds. Our experiments provide a deeper understanding of the fundamental physics of electron transport, while laying the groundwork for technological advances at the nanometer scale,” she says.
Improving the understanding of how molecules transport charge can help in development of next-generation devices, and Venkataraman is on the verge of technological breakthrough. A team in her lab recently substituted layered materials for a gold electrode in a molecular circuit.
“We’ve always used gold metal electrodes to contact molecules in our circuits, but when we switched to using multilayered graphene flakes as one electrode, these devices ended up functioning as diodes, which are the basic building blocks for a transistor,” she explains. That research was published in Proceedings of the National Academy of Sciences this summer.
The next step for Venkataraman is to figure out how to make more interesting circuit components such as switches or optically active elements and build these into circuits. In this process she is also exploring new ways to control the interface between molecules and electrodes.
“We are working on creating molecular devices with varied functionalities that are controlled through chemical design,” she says. To understand the interplay of physics, chemistry, and engineering at the nanometer scale, Venkataraman works with a range of scientists and encourages the graduate students in her lab to get comfortable with collaboration.
“Collaboration is really what makes my research possible and fun,” she says. “Most of my graduate students and I are trained as physicists. We do not know much chemistry, and all the work that we do requires understanding molecular structures to relate them to the electronic functions that we measure. Without our spending hours talking with our chemistry collaborators, none of this would be possible.”
—by Amy Biemiller