Research

Schematic single-molecule device (image: J.W. Ciszek)

Overview

The Natelson Group is interested in nanoscale condensed matter physics, in particular the electrical and magnetic 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.

To explicitly see the impact of quantum mechanics on the properties of matter (e.g. quantum interference contributions to electrical conduction) typically requires small systems (~ 1 micron, or 1/50 the diameter of a human hair) and low temperatures (often < 4.2 K, or -269 degrees Celcius). However, at the atomic size limit quantum effects can dominate even at room temperature.  As systems approach the nanoscale the interplay between quantum effects, the "decohering" influence of the environment, electronic correlations like magnetism and superconductivity, and disorder lead to an impressively rich area of study.

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). (For example, the figure above shows pairs of continuous metal wires on the edge of a semiconductor crystal.  Left to right, the wire widths are 50 nm, 30 nm, and 15 nm, respectively.  The scale bar is 1 micron.)  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 (natelson@rice.edu) and all those interested in this area in general are welcome to look at the reference materials for Doug's special topics course from Fall 2000, (PHYS600: Introduction to Nanoscale Science and Technology, (though beware of typos), or of his nano-sequence, Nanostructures and Nanotechnology I (PHYS533) and II (PHYS534).
 

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 is supported in part by the Robert A. Welch Foundation, The Research Corporation, the Packard Foundation, the National Science Foundation, and 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.

Figure 1. Nanostructure for making atomic-scale Ni junctions

We have been able to make atomic-scale point contacts using both electrochemistry and electromigration methods. We have recently examined atomic-scale contacts between ferromagnetic electrodes. Using electron beam lithography and controlled electromigration, we have been successful in creating junctions ranging from a few channels to tunneling contact. We find large mesoscopic variations from device to device, implying that the precise spin configurations of the "frontier" atoms can differ significantly from those of the bulk electrodes. That work has been reported here, and was supported by an NER grant from the National Science Foundation.

Single-molecule transistors

Figure 2.Schematic single-molecule transistor

We are one of a small number of research groups in the world who have successfully made single-molecule transistors (SMTs). 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).

Figure 3.Level structure of SMT

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. We have also worked with some of our theorist colleagues to show that SMTs have potential as model systems to study quantum criticality and some of the many-body physics relevant in heavy fermion compounds.
• 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

Figure 4.Nanoelectrodes for SERS

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.

Figure 5.Simultaneous single-molecule SERS and conduction

Recently 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.

In our latest results, we have 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 ossibilities!

Organic semiconductors

Figure 6.Array of organic transistors

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).

This project is supported by the NSF and previously by the Robert A. Welch Foundation.

Strongly correlated nanostructures: magnetite

Figure 7.Crystal structure of magnetite. Blue atoms are tetrahedrally coordinated Fe²⁺; red atoms are octahedrally coordinated, 50/50 Fe²⁺/Fe³⁺; white atoms are oxygen.

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.

One example of such a 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.

Figure 8.Hysteretic conduction in a nanoscale magnetite device.

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 hysteresis in the electronic conduction, and we are continuing to investigate this newly found nonequilibrium phase transition.

This work is supported by the Material Sciences and Engineering Division of the Department of Energy's Office of Basic Energy Sciences.

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.

This work had been supported by the DOE and the Packard Foundation.