# Presentation: 444590

## Transistors are semiconductor switches that form the basis of modern electronics technologies.  The ultimate limit of  continued device scaling toward smaller sizes is the single-molecule transistor (SMT).  These devices hold promise for ultrasmall sensor applications, and are also ideal tools to study physics and chemistry at the single molecule scale.  Phenomena that we have examined include electronic detection and tuning of the vibrational properties of single molecules, and the formation of many-body electronic states involving the molecule metal electrodes.

- (a) Schematic diagram of single-molecule transistor and its discrete energy levels; (b) map of conductance *dI/dV*SD (brightness) as a function of source-drain voltage and gate voltage for a SMT, showing a resonant maximum (arrow) that indicates strong electronic correlations with the leads; (c) map of *d*2*I*/*dV*SD2 showing gate-tuning of vibrational energies (arrows).

- (a)

- (b)

- (c)

- source

- drain

- gate

- source

- drain

**Notes:**
It has recently become possible to fabricate three-terminal electronic devices in which the active channel is a single small, organic molecule, shown schematically above in (a).  These single-molecule transistors (SMTs) have electronic properties largely controlled by the highly discrete molecular level spectrum.  Because of the single-nanometer scale of these molecules, both the spacing of the individual quantum states and the electron-electron repulsion that must be overcome to add an electron to the molecule are large energy scales (hundreds of meV).  As a result, large single-electron charging effects may be observed near room temperature.  Combining this with the wealth of binding chemistries from which to choose suggests that SMTs may be useful as ultrasmall sensors.  

Furthermore, these nanoscale devices are ideal tools for examining fundamental physics and physical chemistry problems.  As shown in (b) above, a resonant peak in the conductance appears as a function of gate voltage.  This resonance is the signature of the formation of the Kondo state, a rich many-body electronic state coupling a single spin localized on the molecule (in this case a transition metal complex) with the conduction electrons in the gold leads.  In (c), further analysis shows features in the conduction (black arrows) that correspond to vibrational modes of the molecule.  As a function of gate voltage, the energies of these vibrational modes can be shifted controllably (white arrows), demonstrating electronic control of the mechanical properties of a single molecule.  If one models the molecule’s vibrational properties as balls and springs, this is akin to using an electric potential to adjust the spring stiffness.

## Single-molecule transistors are potentially exciting components of future technologies.  While SMTs are unlikely to be competitive with Si devices for high speed computation, they are well-suited for use as chemical sensors.  Further advances that take advantage of spin degrees of freedom localized to the molecules may enable single-spin electron spin resonance spectroscopy, and possibly spintronic and quantum information applications.  Furthermore, graduate students trained in the research methods of these experiments are positioned at the cutting edge of the future technological workforce.

**Education:**
- Two graduate students and an undergraduate have contributed to this program.  Lam H. Yu will complete his PhD in October and begin a postdoctoral fellowship at NIST in Maryland.  Zach Keane, who characterized the transition metal complexes’ magnetic properties, will take over the project.  The results have been presented by students in talks at national conferences, and have been incorporated into lecture material for the PI’s two-course sequence on nanostructures and nanotechnology.  These results have also been shown to high school students from traditionally underrepresented groups in the sciences and engineering, as part of an ongoing outreach program.

**Societal Impact:**
- Single-molecule transistors are potentially exciting components of future technologies.  While SMTs are unlikely to be competitive with Si devices for high speed computation, they are well-suited for use as chemical sensors.  Further advances that take advantage of spin degrees of freedom localized to the molecules may enable single-spin electron spin resonance spectroscopy, and possibly spintronic and quantum information applications.  Furthermore, graduate students trained in the research methods of these experiments are positioned at the cutting edge of the future technological workforce.

**Notes:**
Understanding the molecular-scale limits of electronic conduction is undoubtedly relevant to the development of future electronics technologies.  When conduction is determined by the chemical properties of a single molecule, sensors that detect single chemical binding events are then possible, opening the door to ultrasmall devices for a variety of sensing applications.  Students trained in the skills needed to fabricate and characterize these devices are positioned to lead the next generation of the technological workforce.

These novel devices, composed of single molecules attached to “wires”, are a conceptually simple system with which to introduce a nonspecialist audience to a host of nanoscale science issues.  High school students with any chemistry coursework can appreciate the size scale of these systems, the importance of quantum mechanics (electronic orbitals) in determining their properties, and the idea that such systems allow novel spectroscopies that can probe single molecules.