****NON-TECHNICAL ABSTRACT**** The transistor is the underpinning of the Information Age. Over the last sixty years, the transistor has been reduced from several centimeters to a few tens of nanometers, with each device now more than 1000 times smaller than the diameter of a human hair. As sizes are reduced further, quantum effects become critically important in determining device properties. At the same time, it becomes increasingly difficult to acquire information about the detailed geometry and material quality of the devices. In the last decade the ultimate limit of this scaling has been reached with the development of single-molecule transistors (SMTs), where the critical channel for current is a small molecule 1-2 nm in size. These devices are excellent tools for examining the physics of conduction at the nanoscale, including the origins of dissipation and the importance of electron-electron interactions. This award supports a project with the goal of extending SMT studies beyond electrical conduction to include measurements of electrical noise. Noise, in the form of fluctuations in the current through the device, is predicted to provide valuable information about both vibrational and magnetic processes within SMTs. Two graduate students will be trained in state-of-the-art nanofabrication methods and will use both low- and high-frequency techniques to measure the noise in SMTs under various conditions, thus preparing them for future academic or industrial careers. If successful the research results will provide new insights into the physics relevant to future technologies, while the work itself will open the door to further new measurements, including the detection and manipulation of single electron spins.
Single-molecule transistors (SMTs) have been developed over the last decade, and have been outstanding tools for examining the physics of conduction at the nanometer scale. SMTs are highly quantum mechanical systems where electron-electron, electron-vibrational, and spin-based interactions can all be strong. One persistent challenge in understanding such structures is the limited information available through DC characterization of electronic conduction. Higher frequency measurements of conduction as well as shot noise can provide much deeper insight into the roles of electron-vibrational and strong correlation effects. Shot noise in particular gives information about the correlations between electron tunneling events. This award supports a project to use both low- and high-frequency methods to measure shot noise in SMTs under various conditions. In SMTs containing unpaired electrons, shot noise will be used to test the theoretical prediction that the effective charge of the electron is modified to a fractional value when transport takes place via the Kondo process. These measurements will lay the groundwork for the eventual development of high frequency ?RF-SMTs? and possible single-molecule electron spin resonance. This program will provide state-of-the-art training of two graduate students in nanoscale science fabrication and measurement techniques thereby preparing them for future careers in academic or industrial research.
It is now possible to construct electronic devices with functional regions as small as a single atom. At this size scale, the quantum nature of electrons can lead to electronic responses that differ greatly from classical devices. This is both a challenge to our basic scientific understanding of the relevant physics, and an opportunity that may lead to the development of novel technologies that leverage these properties. Intellectual merit: One way to learn more about quantum effects in electronic conduction is to measure not just the current as a function of voltage, but to look at noise – the fluctuations in the current. With zero average current, the noise is related to the thermal fluctuations of the electrons, while when a device is driven to nonzero current (by a battery, say) the noise is deeply connected to the timing of electron motion. The noise is parametrized by a factor F that compares measured noise with that expected for truly independent electrons. In this program we developed a radio-frequency method of measuring this noise at comparatively high speed, essential for studying the properties of structures that vary in time. While we originally planned to look at single-molecule transistors, we realized during our experiments that first we needed to understand the response of simpler structures, atomic-scale metal wires, formed either by moving a metal tip in and out of contact with a metal film, or by a patterning process similar to that used in fabricating computer chips. At the atomic scale, certain arrangements of atoms are more stable than others, an effect that is detectable through peaks in histograms of the measured conductance. At those preferred values of conductance, the theoretical expectation is that the noise should be comparatively suppressed, a quantum effect previously only seen at cryogenic temperatures. With our approach, we demonstrated for the first time that this effect persists up to room temperature. The noise also provides a means of looking at heating of the electrons and the transfer of energy from the electrons to the vibrational motion of the atoms themselves. In subsequent experiments, the signatures of electronic heating and the interactions of the electrons with vibrations were observed, as well as a remarkable sensitivity of the noise to the precise details of the atomic arrangement. These results were reported in five publications, as well as a large number of colloquia, seminars, and invited talks at international conferences. Broader impacts: The origins of intrinsic noise and the nature of heating and dissipation in nanoelectronic devices is of critical importance to the future of ultrascaled electronics of the sort associated with next-generation computer chips. While the structures in this work are not themselves employed in that technology, our experiments address physics that is directly relevant to future electronics technologies. Over its duration, this project provided support for the research training of two graduate students (one completing the masters and moving on to doctoral candidacy; the other completing the PhD); these students were also educated in spoken and written communications skills, and thus prepared for careers in the nation’s science and technology workforce. Thanks in part to the support of this award, the PI was also able to work with two undergraduate researchers and a visiting exchange student from Japan. This work and related material was presented to the greater public through interactions with the Children’s Museum of Houston, STEM education projects involving Houston-area high schools, public lectures ("Nerd Nite Houston"), and science blogging by the PI (http://nanoscale.blogspot.com). This award also encouraged the development of a forthcoming graduate level introductory text on nanostructures and nanotechnology, to be published by Cambridge University Press.
