The Natelson Group is interested in nanoscale condensed matter physics, in particular the electronic, magnetic, and optical properties of systems with characteristic dimensions approaching the atomic scale. A variety of unusual phenomena arise in this size regime, and an understanding of the properties of matter on these scales will likely be essential for future technologies.
Imagine taking some nanoscale structure and somehow hooking it up to a battery via a couple of wires. As the voltage across the nanosystem is increased, many things can happen. An electric field develops across the structure, and charge rearranges itself in response to that field. When there is conduction, electrons are driven from one side of the nanosystem to the other, pushing the distribution of electrons (as a function of both position and energy) out of equilbrium. Those electrons interact with each other, and energy can pass between them; the electrons can also deposit energy into the vibrational motion of the atoms, driving those degrees of freedom out of equilibrium. The electrons can interfere quantum mechanically, though their coherence (and therefore the importance of quantum interference) degrades as they interact with their environment. The spins of the electrons can become entangled with each other. All of this takes place on very short time and distance scales, and somehow leads to the macroscopic world we see all around us. Our group tries to understand the physics at work here, through experiments and their analysis.
Techniques now exist for controllably fabricating structures with characteristic lengths below 10 nm, through both "top-down" methods (combining electron beam lithography, directional etching, and metal deposition) and "bottom-up" approaches (chemical fabrication, self-assembly). We can now examine "traditional" solid state systems at previously unexplored scales by making samples in this newly accessible regime. Further, we can develop tools for studying novel materials such as organic semiconductors and other molecular and molecular-scale objects.
In addition to learning a lot of neat physics, people in this research group develop skills in micro- and nanofabrication, sensitive electrical measurements, various microscopy techniques, semiconductor physics, low temperature physics, and vacuum systems. Graduate students or interested undergraduates are encouraged to send email (email@example.com).
Single-molecule electronics and optics
It is now possible to examine the electronic, magnetic, and optical properties of materials down to the atomic scale in certain circumstances. These systems are of fundamental scientific interest and are likely to be relevant to next-generation technologies.
This research has been supported in part by the Robert A. Welch Foundation, The Research Corporation, the Packard Foundation, the National Science Foundation, the W. M. Keck Program in Quantum Materials at Rice.
Atomic-scale metal junctions
In some ways the simplest kind of nanoscale device is an atomic-scale junction between two pieces of metal. Imagine breaking a metal wire into two pieces. At the last instant before the wire breaks, the two sides are linked by an atomic-scale connection. The electronic conduction of such a junction are dominated by quantum effects even at room temperature. For metals with mostly s-type conduction electrons (e.g., Au) the conductance of an atomic point contact is given by 2e2/h, where e is the electronic charge and h is Planck's constant.
Lately we have started using high frequency (200-500 MHz) techniques to measure noise in atomic-scale structures. Current noise (mean square fluctuations about the average current) can reveal much about correlations between the motions of electrons. If electrons transit a system (without inelastic scattering) with some characteristic rate but are otherwise uncorrelated (Poisson statistics), the current noise is white (frequency independent out to some cutoff) and given by 2eI, where I is the average current. Adding in correlations changes this result. For example, perfectly coordinated electron equally spaced in time would suppress the current noise all the way to zero. Conversely, electrons travelling in bunches rather than independently would enhance the noise. Noise in quantum nanostructures is known to show suppression under particular circumstances (see here for example). We have been making such measurements (as shown in Fig. 1, where particular quantized values of the conductance (blue) correspond to suppressed noise (red)), with an eye toward examining physical effects predicted to lead to enhanced noise, particularly in molecular devices. Remarkably, we have been able to see this inherently quantum mechanical suppression of shot noise even at room temperature, as reported here. These experiments look at physics relevant to dissipation of energy and electronic correlations at the atomic and molecuar scale.
Recently, in collaboration with the Untiedt group at Alicante, we have found evidence for Kondo physics in atomic-scale junctions between ferromagnetic metals. This is rather surprising, since Kondo physics involves antiferromagnetic process that couple a local electronic spin to the spins of conduction electrons. It appears that the undercoordination of the atom(s) at the contact region modifies the local electronic structure, making this physics possible.
We are one of a relatively small number of research groups in the world who have successfully made single-molecule transistors (SMTs). A readable article for nonexperts is here (pdf). SMTs are three-terminal devices, with source and drain electrodes to pass current into and out of a molecule, and also a gate electrode to shift molecular level energies up or down relative to the source and drain. These structures generally act like single-electron devices, shown schematically at right. In the limit shown, electronic conduction through SMTs is strongly affected by the single-particle level spacing (the energy required to promote an electron from the highest occupied level to the lowest unoccupied level) and the Coulomb charging energy (the energy needed to add an additional electron to the molecule).
In SMTs, the small molecular size implies that both of these energy scales can be hundreds of meV, vastly higher than in conventional metal or semiconductor single-electron devices. As a result, physical chemistry issues such as molecular vibrational modes and conformational changes can become relevant. Depending on the strength of the molecule/electrode coupling, higher order tunneling processes can strongly affect conduction as well. In molecules with unpaired spins, magnetic effects can result in the development of strongly correlated electronic states (e.g.the Kondo resonance) that span the device. The details of the contact also determine the relative alignment of the electrode and molecular levels, with major implications for transport. The interplay between all of these issues means that conduction through SMTs can exhibit a rich variety of phenomena.
Prof. Natelson has written an extensive review article about single-molecule transistors. Please contact Prof. Natelson if you would like a copy. Similar matters are discussed in these two papers (1, 2).
We have used these devices to study several interesting physics and physical chemistry problems:
- We fabricated SMTs incorporating individual C60 molecules. We were the first group to observe Kondo physics in C60, as well as strong interplay between the Kondo resonance and vibrational levels.
- In SMTs using transition metal complexes provided by our chemist colleagues, we implemented gate-modulated inelastic electron tunneling spectroscopy at the single molecule level. We saw nontrivial changes in vibrational levels when the vibrational energy and the energetic cost to make an electronic transition became comparable. This gate modulation of vibrational energies is a nonperturbative effect that we are studying further.
- In SMTs with the same complexes, we have examined Kondo physics in detail. These devices exhibit very strong Kondo physics, with a characteristic Kondo temperature on the order of 70 K. Furthermore, the gate dependence of the Kondo temperature is (anomalously) very weak, demonstrating that these systems exhibit many-body physics different from that in standard semiconductor devices. This work has been reported here. Most recently, we have looked in detail at the voltage and temperature scaling of Kondo physics in molecular devices. We have found (here) that Kondo resonances in our molecular devices do scale with the same functional form seen in Kondo measurements on semiconductor quantum dots (even though our Kondo temperatures differ from those in GaAs dots by a factor of 100), though there are systematic differences in the numerical details that remain unexplained. Put simply, for a given temperature dependence, the Kondo resonances in the molecular case are considerably broader in voltage than the semiconductor case. We have written a review article summarizing the state of the art in Kondo physics in molecular transistors.
- We also used SMTs to examine the mechanism of bistable, hysteretic conduction observed in some molecules. We have observed the hysteretic conductance switching, and found essentially no dependence of the switching properties on gate voltage. This effectively rules out one suggested switching mechanism (bias-driven polaron formation) in this particular molecule.
Surface-enhanced Raman spectroscopy and Nano-optics
Raman spectroscopy is a common chemical characterization technique in which incident light interacts inelastically with a system, either losing energy toa vibrational or electronic mode of the system ("Stokes scattering"), or gaining energy from the system ("anti-Stokes scattering"). Raman cross-sections for single molecules tend to be small (~ 10-29 cm2). However, nanostructured metal surfaces can act like little optical antennas due to electronic excitations called plasmons. The local electric field in the presence of plasmons can be enhanced by a factor g over that of the incident light. This translates into an enhancement of Raman emission by roughly g4 for molecules in the region of this near-field effect. The result is surface-enhanced Raman scattering (SERS), which can have single-molecule sensitivity.
In 2007 we demonstrated that the nanoscale electrodes used for the SMT experiments are outstanding plasmonic antennas and therefore wonderful substrates for SERS. This work was picked up by Nature Photonics.
(Here and here), we successfully performed simultaneous electronic transport and SERS measurements on single molecules. This confirms that transport is through the molecule of interest (via the unique Raman spectroscopic signature) and demonstrates that we can make single-molecule sensitivity Raman "hotspots" in predefined locations. This opens up many exciting experimental possibilities!
When light shines on a metal nanojunction and excites the local plasmon modes responsible for the enhancement, the plasmons lead to a voltage across the nanojunction, oscillating at optical frequencies (exceeding 1014 Hz). Combining optical and electronic transport measurements in junctions without molecules, we have used optical rectification to determine this voltage experimentally. Using the simultaneously measured tunneling conductance, we can then infer quantitatively the enhanced electric field in the junction. Consistent with our Raman results, we find that field enhancements can exceed 1000x.
Most recently, we have used Raman scattering to examine the pumping of vibrational and electronic populations as current flows through a molecule-containing junction. This work demonstrates that it is possible to access experimentally the energetic distributions of electrons and vibrational modes in driven junctions, in situ, at the single molecule scale. While there has been much theoretical discussion of these distributions, attaining experimental information about the situation is very difficult.
We have begun experiments to examine other nano-optical phenomena in these plasmonic structures. This includes efforts to examine optical effects in semiconductor nanoparticles.
Organic semiconducting materials are widely recognized as having tremendous potential for certain electronics applications. The charge transport properties of such materials are determined by both the microstructural arrangement of molecules (as in inorganic semiconductors) and by the chemical structure of the molecules themselves.
The optical microscope image above shows an array of interdigitated Au and Pt electrodes fabricated by e-beam lithography on a degenerately doped silicon wafer coated with a 200 nm insulating oxide layer. The electrode spacing varies, allowing us to distinguish between bulk and contact effects. On top of the electrodes is a solution-cast film of poly(3-hexylthiophene) (P3HT), a commercially available organic semiconducting polymer.
This program examines such materials over a broad range of temperature, carrier density, and device size. We want to understand the transport mechanisms and contact phenomena in these materials, and how those properties evolve as device size is reduced toward the molecular scale. Our accomplishments include:
- We have examined the relationship between semiconductor mobility and contact resistance in devices based on P3HT and Au electrodes. We have achieved record mobilities in this system (approaching 1 cm2/Vs) and demonstrated that, for the diffusion-limited injection regime, contact resistance is inversely proportional to the mobility over four decades in mobility.
- We have developed a method for extracting the current-voltage characteristics of just the metal-organic contact.
- We have studied the effects of dopant concentration on the injection process. We found that the (unintentional) concentration of acceptor dopants is reduced by annealing at modest temperatures in vacuum. As doping is lowered, Au becomes a poor, nonOhmic contact, while injection from Pt remains Ohmic and relatively good. This correlates with UPS measurements that show better alignment between the Pt Fermi level and the P3HT valence band than in Au.
- We have also shown that surface chemistry may be used to manipulate those metal/organic interfacial energetics. With self-assembly of the appropriate work function-raising molecule, Au electrodes may be modified so that their injection properties act like those of Pt. This is one route to optimizing contacts in organic FETs.
Current efforts are aimed at understanding single-molecule-scale effects in P3HT, as well as THz probes of carrier dynamics (collaboration with the Mittleman group) and nm-scale mapping of metal/organic interfaces (collaboration with the Kelly group). Recently we have found good evidence that interfacial charge transfer at the Pt/P3HT interface effectively dopes the P3HT with mobile holes very locally.
Strongly correlated nanostructures
Most experiments examining the electronic properties of nanostructured materials have focused on simple metals and semiconductors. We are very interested in applying nanoscale transport techniques to strongly correlated materials, systems in which the simple single-electron approach of conventional band theory fails. Such nanostructure-based experiments, while challenging, can (1) apply large electric fields without the need for large voltages, discriminating between different physical processes; (2) probe inhomogeneous systems on a scale smaller than their inhomogeneity; and (3) enable sensitive studies of noise and contact effects not readily performed in macroscopic structures. We published a review article about this exciting area here.
One example of a strongly correlated material is magnetite, Fe3O4, also known as lodestone. Known for thousands of years, magnetite is an example of a strongly correlated transition metal oxide. At room temperature magnetite is moderately conducting, but as T is reduced, its resistivity increases (unlike that of a "good" metal). When cooled below around 120 K, magnetite goes through a first-order phase change (the Verwey transition) and becomes much more insulating. The mechanism of this transition and the nature of the insulating state have been controversial for nearly 70 years.
In our recent studies of electronic conduction in magnetite nanostructures, we found that at temperatures below the Verwey transition it is possible to kick the material out of the insulating state and back into a more conducting state via electric fields. The result is dramatic switching in the electronic conduction, and we are continuing to investigate this newly found nonequilibrium phase transition. Recently we have found that while the switching from low- to high-conductance is driven by electric field, the hysteresis shown in the figure is a signature of local heating, and may be eliminated by performing pulsed measurements.
We have also been applying nanostructure techniques to other correlated materials, including vanadium dioxide (VO2). This material has a dramatic metal-insulator transition at 67oC, and may be grown in nanowire form. We have some exciting work coming out that looks at ways of engineering this transition to better understand the physics behind it.
This work is supported by the Material Sciences and Engineering Division of the Department of Energy's Office of Basic Energy Sciences.
There are a number of systems that have electronic conduction properties that may be switched, through voltage or current cycling, between two states, a high-conductance "on" state, and a low-conductance "off" state. As you might imagine from the terminology, there is much industrial interest in employing such systems as switches or nonvolatile memories. Hewlett Packard has coined the term "memristor" to describe such a device, though the phenomenon itself is not surprising. The physics challenge is distinguishing between the multiple mechanisms that can lead to similar observed properties. Collaborating with Prof. James Tour and Prof. Lin Zhong, we have found that silicon oxide, a material often treated cavalierly as an inert insulator, can show this kind of switching, as described here. Here is a recent review of this and other phenomena related to ion motion in nanostructures.
Quantum coherence effects
Electrons are quantum mechanical objects, yet in daily experience one does not worry about quantum interference effects when, for example, turning on the lights. The reason for this is decoherence. As electrons interact inelastically with their environment (including other electrons), their quantum mechanical phase relationships become scrambled on a time scale called the coherence time (and a corresponding distance scale, the coherence length). At room temperature, this length scale is ~ 1 nm. At low temperatures, however, it can reach easily accessible scales and produce significant quantum corrections to the electrical properties of conducting systems.
Information about decoherence processes must be inferred from measurements of those corrections. An important outstanding question is, Do different quantum corrections probe the same decoherence physics? In the past we have concentrated on measuring universal conductance fluctuations and weak localization.
We have used these measurements to probe the nature of quantum-enhanced noise in normal metal nanowires, including the surprising effects of surface chemistry on the source and distribution of that noise. These measurements also revealed interesting magnetic properties of partially oxidized titanium.
We have also performed similar measurements in ferromagnetic metal nanowires as well as InMnAs dilute magnetic semiconductor nanowires. Quantum coherence effects in ferromagnetic systems remain relatively unexplored experimentally, and ferromagnetism leads to new collective modes (e.g., spin waves) that may profoundly affect electronic coherence.