Measurement of Single-Molecule Conductance - public.asu.edu

Annu. Rev. Phys. Chem. 2007.58:535-564. Downloaded from arjournals.annualreviews.org by ARIZONA STATE UNIV. on 01/10/08. For personal use only.

ANRV308-PC58-20

ARI

22 February 2007

11:34

Measurement of Single-Molecule Conductance Fang Chen, Joshua Hihath, Zhifeng Huang, Xiulan Li, and N.J. Tao Department of Electrical Engineering and Center for Solid State Electronics Research, Arizona State University, Tempe, Arizona 85287; email: [email protected]

Annu. Rev. Phys. Chem. 2007. 58:535–64

Key Words

First published online as a Review in Advance on November 29, 2006

molecular electronics, break junction, quantum point contact

The Annual Review of Physical Chemistry is online at http://physchem.annualreviews.org

Abstract

This article’s doi: 10.1146/annurev.physchem.58.032806.104523 c 2007 by Annual Reviews. Copyright All rights reserved 0066-426X/07/0505-0535$20.00

What is the conductance of a single molecule? This basic and seemingly simple question has been a difficult one to answer for both experimentalists and theorists. To determine the conductance of a molecule, one must wire the molecule reliably to at least two electrodes. The conductance of the molecule thus depends not only on the intrinsic properties of the molecule, but also on the electrode materials. Furthermore, the conductance is sensitive to the atomiclevel details of the molecule-electrode contact and the local environment of the molecule. Creating identical contact geometries has been a challenging experimental problem, and the lack of atomiclevel structural information of the contacts makes it hard to compare calculations with measurements. Despite the difficulties, researchers have made substantial advances in recent years. This review provides an overview of the experimental advances, discusses the advantages and drawbacks of different techniques, and explores remaining issues.

535

ANRV308-PC58-20

ARI

22 February 2007

11:34

1. INTRODUCTION The ability to measure and control electrical current through a molecule is a critical requirement toward the goal of building electronic devices using individual molecules (1, 2). This ability also offers us a unique opportunity to understand charge transport, an important phenomenon that occurs in many chemical and biological processes (3, 4), on a single-molecule basis. Furthermore, it allows us to read the chemical information of a single molecule electronically (5), which opens the door to chemical and biosensor applications based on the electrical detection of individual molecular binding events (6). The most fundamental quantity that describes the electrical properties of a bulk material is conductivity, based on which materials are often divided into conductors, insulators, and semiconductors. Conductivity is defined as σ = (I/V) × L/A, where I is the electrical current, V is the applied bias voltage, L is the length, and A is the cross sectional area of the material. For a small molecule, A and L are difficult to define precisely, and a more well-defined quantity is the conductance, G, given by G = I/V. So a basic question in molecular electronics is, what is the conductance of a single molecule? To determine the conductance of a molecule, one must first bring it into contact with at least two external electrodes (Figure 1). The contact must be robust, reproducible, and able to provide sufficient electronic coupling between the molecule and the electrodes (7–11). One way to meet the contact requirements is to attach the molecule covalently to the electrodes via proper anchoring groups at two ends of the molecule. Clearly, the measured conductance depends not only on the molecule, itself, but also on the properties of the electrodes and the atomic-scale molecule-electrode contact geometry (12–17). Another factor that may affect the measured conductance is the local environment of the molecule, which includes solvent molecules (vacuum or air), pH, and ions (18, 19). Finally, an electron traversing through the molecule


Molecular electronics: science and technology related to the understanding, design, and fabrication of electronics devices based on molecules Contact geometry: the atomic-scale geometry of an electrode, and the binding site and orientation of a molecule on the electrode

a

b I

I

Vbias

Vbias Figure 1 (a) Current through a molecule covalently bound to two electrodes. (b) Current through a metal atom attached to two electrodes made of the same metal.

536

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

may exchange energy with the vibration modes of the molecule or surrounding solvent molecules (20, 21). Consequently, the measured conductance may be sensitive to temperature (22–24). Precisely controlling these measurement details, especially the molecule-electrode contacts, has been a difficult task, which is a major reason for the large disparities between different measurements for even a relatively simple molecule, such as alkanedithiols (25, 26). In this chapter, we review recent experimental advances and discuss remaining issues in measuring single-molecule conductance. A related problem is to determine the conductance of a single metal atom or a linear chain of metal atoms bridged between two electrodes made of the same metal as the atoms in the chain (27–32). This system is known as a quantum point contact. For metals, such as Au, the conductance of a quantum point contact is close to G0 = 2e2 /h = 77.4 μS, where e is the electron charge, h is the Planck constant, and S is the conductance unit. G0 is the conductance quantum unit corresponding to 100% transmission of electrons from one electrode to the other through the point contact. Therefore, the conductance of a molecule given in units of G0 tells us how conductive the molecule is in comparison with a metal atom, which is why we use G0 as the unit of conductance here. Molecular electronics is a diverse and rapidly growing field, and many excellent reviews have appeared in recent years, covering general concepts (4, 33–36), theoretical aspects (21, 37, 38), and experimental methods (39–42). In the present review, we mainly focus on the measurements of single molecules covalently bound to two electrodes. We refer the readers interested in molecule-film measurements to recent reviews by Metzger (42) and Wang et al. (41). The remainder of the chapter is arranged as follows: Section 2 provides a brief overview of experimental tools that measure the conductance of molecular thin films, and Section 3 describes techniques and methods for single-molecule conductance measurements, in which we discuss both the advantages and difficulties of different techniques. Section 4 addresses some critical issues that affect single-molecule conductance, and Section 5 provides a summary and outlook.

Molecule-electrode contact: contact between a molecule and an electrode provides sufficient and stable electronic coupling between the molecule and the electrode Quantum point contact: a small constriction between two conducting regions through which electrons traverse ballistically and experience a quantum waveguide effect Conductance quantum unit: a basic unit of conductance given by G0 = 2e2 /h, where e is the electron charge and h is the Planck constant

2. MEASUREMENTS OF MOLECULAR FILMS To date, many experimental techniques have been developed to measure and control current through molecules. We can divide these techniques into two broad categories: molecular film and single-molecule measurements. We briefly describe the first category in this section, whereas we discuss single-molecule conductance measurements in more detail in Section 3.

2.1. Bulk Electrodes The basic idea of the bulk electrode approach is to sandwich a molecular film, either a monolayer or multiple layers, between two electrodes. The molecules are typically first immobilized on one electrode surface, via self-assembly or Langmuir-Blodgett methods (40, 42). Both self-assembly and Langmuir-Blodgett methods provide ordered molecular films, which are highly desired for successful measurement and data

www.annualreviews.org • Single-Molecule Conductance

537

ANRV308-PC58-20

ARI

22 February 2007


Molecular junction: a molecule is attached to two electrodes to form an electrode-moleculeelectrode structure

11:34

interpretation. A second electrode is then placed on top of the molecular film, and the electrical current between the two electrodes is measured as a function of bias voltage. The placement of the top electrode is often the most difficult part of the measurement, and many methods have been developed (43). One is to evaporate a metal layer onto the molecular film in vacuum. To avoid a short circuit between the top electrode and the bottom electrode, one must ensure that energetic metal atoms do not damage the molecular film and the deposited metal does not penetrate into the molecular film via defect sites to reach the bottom electrode (44, 45). To minimize the short-circuit problem, Akkerman et al. (46) put a conducting polymer layer onto the molecular film before the deposition of the top electrode. Another method is to print a metal layer onto the molecular film by transferring a metal film evaporated on a stamp via direct contact (47, 48). Electrodeposition methods have also been investigated (49–51). For example, Manolova et al. (52) used a 4,4 -dithiodipyridine-modified Au electrode to allow Pd or Pt ions to adsorb preferentially onto the pyridine moiety of the molecules. The adsorbed metal ions were then reduced electrochemically to metallic Pd or Pt that could serve as the second electrode in a metal ion-free acidic solution (52). Angle-resolved X-ray photoelectron spectroscopy showed that the second electrode was indeed on top of the molecular film without penetrating into the molecule film via defect sites. Instead of using a solid metal as the top electrode, an alternative approach is to place a drop of mercury onto the molecular layer (53–57). The mercury approach is relatively simple and has shown high device yields.

2.2. Nanoelectrodes The molecular film measurements described above involve many molecules, which can be reduced using nanoscale electrodes. For example, a conducting atomic-forcemicroscope tip with a radius of curvature on the nanometer scale can serve as the top electrode to measure conductance of a relatively small number of molecules (58, 59). Another method is the cross-wire junction, which sandwiches a molecular layer between two perpendicular metal wires (60–62). The contact area between the two wires is small, so a relatively small number of molecules is measured. One may also reduce the number of molecules by confining the molecular film inside a nanopore fabricated in a thin solid membrane (63, 64). One side of the membrane is first covered with a metal layer, and molecules are allowed to form a monolayer on the metal surface inside of the nanopores. A second metal layer is then deposited on top of the molecular film in the nanopores. Nanopores can also be used as templates to fabricate nanoscale metal-molecule-metal junctions (65). Metal is first deposited electrochemically into the nanopores in an alumina membrane to partially fill up the nanopores. Sample molecules are then allowed to adsorb on the metal surface inside of the nanopores, which is followed by the chemical deposition of a top metal layer into the nanopores to form nanoscale molecule junctions. After dissolving the alumina, these molecular junctions in a solution are assembled onto electrode arrays for conductance measurements.

538

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

The molecular film measurements have played an important role in the understanding of the electron-transport properties of molecules (66, 67) and for demonstrations of many device functions, including rectification (44, 68–70), molecular switching (62, 71, 72), and negative differential resistance effects (56, 63, 73). When applying these techniques to a particular sample molecule, one needs to consider the following factors: First, not all the molecules of interest can form well-ordered films, which limits the choice of molecules for the studies. Second, molecular films often have defects, and additional defects may be created during the fabrication of the top electrode (51) and application of bias voltage (61). It is important to ensure that the defects and electromigration-induced effects do not dominate the measured conductance. Third, in a close-packed film, a molecule interacts with the neighboring molecules, which may suppress its conformational change and affect its electron-transport properties (74). As a result, the average conductance per molecule calculated from the molecular film measurements may not be directly comparable with single-molecule conductance measurements. Finally, the molecules in the films are not exposed to the outside world, which makes it difficult to introduce a gate electrode and prevents one from studying molecular binding processes between the sample molecules and analytes.

Electromigration: transport of material caused by the movement of the ions in a conductor owing to the momentum transfer between conducting electrons STM: scanning tunneling microscopy AFM: atomic force microscopy

3. MEASUREMENTS OF SINGLE MOLECULES To determine the conductance of a single molecule, one must (a) provide a signature to identify that the measured conductance is a result of not only the sample molecules, but also a single sample molecule, (b) ensure that the molecule is properly attached to the two probing electrodes, and (c) perform the measurement in a well-defined environment. The current methods fall into three categories: scanning probe methods, fixed electrodes, and mechanically controlled molecular junctions.

3.1. Scanning Probe Techniques Scanning tunneling microscopy (STM) has played a unique role in the field of molecular electronics (75). This is because it allows one to image individual molecules adsorbed on a conductive substrate with submolecular resolution. By placing a scanningtunneling-microscope tip over a molecule, one can perform tunneling spectroscopy measurements on the molecule (76–78). In addition, the tip can be used to manipulate atoms and molecules on surfaces (79). A closely related technique is atomic force microscopy (AFM). Although AFM has typically a lower resolution than STM, it measures both the mechanical and electrical properties of molecules when a conducting tip is used (80, 81). To date, STM and AFM have revealed many interesting phenomena important to the development of molecular electronics. Examples include reversible redox switching (82–85), an electromechanical molecular amplifier (86), the negative differential resistance-like effect (59, 87–89), molecular switching (90, 91), and the study of field effects on electron transport in molecules (92). In the STM or AFM method, a molecule adsorbs onto a substrate to form a well-defined molecule-substrate contact, but the tip-molecule contact is often less well defined, which prevents one from determining the absolute conductance of the


539

ANRV308-PC58-20

ARI

22 February 2007

11:34

a

b Tip

Tip


Metal particle

Substrate

Substrate

Figure 2 (a) Scanning tunneling microscopy (STM) study of electron transport through a target molecule inserted into an ordered array of reference molecules. (b) STM or conducting atomic force microscopy (AFM) measurement of conductance of a molecule with one end attached to a substrate and the other end bound to a metal nanoparticle.

molecule. One way to reduce this problem is to embed a molecule of interest into the matrix of another, often less conducting molecule (Figure 2a). Bumm et al. (90) used this approach to study current through molecular wire candidates inserted into an insulating alkanethiol monolayer and observed much higher current through the molecular wires than through the surrounding alkanes. Tao (82) studied Fe-porphyrin coadsorbed in the ordered array of porphyrin molecules in an electrochemical cell. Fe-porphyrin is redox active, whereas porphyrin is similar in structure but redox inactive. The work shows that the current through the redox active molecule is much higher than that through the redox inactive molecule when the electrode potential is tuned near the redox potential of Fe-porphyrin. Another STM approach demonstrated by Langlais et al. (93) is to attach one end of a molecule to a step edge on a metal substrate. The STM image of the molecule and the metal substrate provides spatially resolved electronic penetration of the metallic electronic states from the step edge into the molecule, which the researchers used to determine the electron tunneling decay constant along the molecule. A third approach is to attach one end of a molecule onto a substrate and the other end to an Au nanoparticle covalently (Figure 2b). One then places a scanningtunneling-microscope tip (94) or a conducting atomic-force-microscope tip (7) on top of the nanoparticle. Because Au nanoparticles are typically coated with an organic surfactant layer, the tip and the nanoparticle form a tunneling junction. Consequently, the nanoparticle behaves similar to a Coulomb island, and the electron transport between the tip and the substrate via the nanoparticle exhibits Coulomb blockade effect (7, 94). By fitting the measured I-V curves with the Coulomb blockade model, one can extract the conductance of the sample molecule between the nanoparticle and the substrate (95, 96).

540

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

Ho et al. (97) recently made a significant step toward the goal of creating a metalmolecule-metal junction with atomic-scale precision. They assembled a copper(II) phthalocyanine molecule bound to two Au atomic chains on an NiAl(110) surface by manipulating individual Au atoms and copper(II) phthalocyanine molecules with a scanning-tunneling-microscope tip. The spatially resolved electronic spectroscopy revealed that the electronic states of the metal-molecule-metal junction change significantly when varying the number of Au atoms in the chains one by one. The NiAl(110) is conducting, so the electronic coupling between the molecule and the substrate is strong. To reduce the electronic coupling, Repp et al. (98) introduced an insulating layer of NaCl between the sample molecule (pentacene) and a Cu substrate. By manipulating an Au atom with a scanning-tunneling-microscope tip, they created an Au-pentacene contact and studied the dependence of the molecule’s electronic states on the formation of the Au-molecule contact (Figure 3). These STM studies not only provided the atomic-scale structural information of the metal-molecule contacts but also determined the mixing of the electronic states of the molecules with those of the metal electrodes.

Figure 3 Making and breaking a chemical bond between a single pentacene molecule and an Au atom on an NaCl bilayer supported by a Cu(100) substrate. (a) Scanning tunneling microscopy (STM) image showing the molecule and the Au atom before bond formation. (b) Image showing a molecule-metal complex (6-Au-pentacene) after resonant tunneling through the lowest unoccupied molecular orbital of the pentacene molecule. (c) Calculated geometry of the adsorbed 6-Au-pentacene complex. C, H, and Au atoms are represented by gray, white, and gold spheres, respectively. Reprinted with permission from Reference 98.


541

ANRV308-PC58-20

ARI

22 February 2007

11:34


In both examples described here, the metal electrodes, consisting of a single or several Au atoms, are too small to develop full continuous density of states of metallic character. The coupling between the molecule and metal substrate is reduced by the thin NaCl layer, but it is still finite as it is needed for STM imaging. Therefore, the results may not be compared directly with the molecular conductance measurements that use large or bulk metal electrodes. Another challenge is to connect the metal electrodes to two external electrodes to carry out electron-transport measurements on the molecular junctions created by STM.

3.2. Fixed Electrodes A conceptually simple method to measure single-molecule conductance is to fabricate a pair of facing electrodes on a solid substrate and then bridge the electrodes with a molecule terminated with proper anchoring groups that can bind to the electrodes (Figure 4a). A key requirement of this method is to fabricate electrodes with a molecular-scale gap, which has been a difficult task. This is because most molecules are a few nanometers or less, and fabrication of such a small gap between electrodes is beyond the reach of conventional micro- and nanofabrication facilities. Unconventional

Figure 4

a

Schematic illustrations of single-molecule conductance studies using different methods. (a) A single molecule bridged between two electrodes with a molecular-scale separation prepared by electromigration, electrochemical etching or deposition, and other approaches. (b) Formation of molecular junctions by bridging a relatively large gap between two electrodes using a metal particle. (c) A dimer structure, consisting of two Au particles bridged with a molecule, assembled across two electrodes.

Source

Drain Oxide Gate

b Metal particle Electrode

Electrode Substrate

c Au

Au

Electrode

Electrode Substrate

542

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

fabrication techniques—such as electromigration (99, 100), electrochemical etching or deposition (101–104), and other novel methods (105–109)—have been developed to create electrodes separated with a molecular-scale gap. One attempt to avoid the requirement of molecular-scale gaps is to use metal nanoparticles as bridges (Figure 4b). This technique starts with a pair of electrodes separated with a gap that is much larger than a sample molecule. After covering the electrodes with a layer of sample molecules, one drives a metal nanoparticle with a diameter greater than the electrode gap into the gap to bridge the two electrodes, either using dielectrophoresis (110) or a magnetic field when Au-coated Ni particles are used (71). This results in two metal-molecule-metal junctions in series, which may complicate the data analysis. Dadosh et al. (111) reported another strategy based on synthesizing a dimer structure, consisting of two Au nanoparticles connected by a dithiolated organic molecule, and electrostatically trapping the dimer structure between two metal electrodes (Figure 4c). Because the fixed electrodes are supported on a solid substrate, the molecular junctions are mechanically stable, which is ideal for studying electron transport through molecules as a function of temperature, magnetic field, and other external parameters. Another advantage of this approach is that one can use the substrate (e.g., heavily doped Si) as a gate electrode to control the electron transport through the molecules. Researchers have applied this approach to study various interesting phenomena, including single-electron charging (99, 100, 107, 112) and Kondo effects (99, 100, 113) in molecules at low temperatures. Further improvements are desired to address the following issues. First, the deviceto-device variation is large, which calls for statistical analysis. However, the fabrication yield is usually rather low, making it difficult to perform statistical analysis (104, 105). Second, for redox molecules, the single-electron charging effect was used as a signature to identify measurements attributed to a single molecule (99, 100, 107, 112). But, in general, it is difficult to determine how many molecules are bridged across the electrodes and how many contribute to the measured current. Because metalnanoparticle complexes often form during the fabrication processes of the molecularscale gaps, it is important to distinguish that the measured transport properties are a result of the sample molecule rather than the nanoparticles or a nanoparticlemolecule complex (114). Third, even though the molecules have two linker groups, it is difficult to determine if the molecules are indeed covalently bound to the two electrodes. Finally, the structure of the electrodes and the position of the molecule are not precisely known, so a technique that can provide atomic-scale structural information of the molecular junction is highly desired.

Redox molecule: molecule that can be reversibly oxidized or reduced via electron transfer between the molecule and electrodes PZT: piezoelectric transducer

3.3. Mechanically Formed Molecular Junctions It is difficult to fabricate electrodes separated with a gap that can fit a sample molecule precisely. This difficulty may be overcome by bringing two electrodes together or separating them apart mechanically. Using a piezoelectric transducer (PZT) or other mechanical actuation mechanisms, one can control the gap with subangstrom precision, making it possible to fabricate molecular junctions and measure the conductance


543

ANRV308-PC58-20

ARI

22 February 2007

11:34

of various molecules. Several techniques based on this concept have been developed, which are described below.


MCBJ: mechanically controllable break junction

3.3.1. Mechanically controllable break junction. The basic principle of the mechanically controllable break junction (MCBJ) method (115, 116) is to break a thin metal wire supported on a solid substrate into a pair of facing electrodes (32). The process is controlled by bending the substrate with a mechanical actuator, such as a PZT and stepping motor (Figure 5a). An MCBJ device can be built manually by fixing a metal wire with a notch near the middle of the wire (116). An alternative method is to create a nanobridge connecting two microelectrodes on a substrate using electron-beam lithography (Figure 5b) (117). Because the bending is achieved by fixing two ends of the substrate while pushing the middle part of the substrate vertically, the vertical movement of the PZT is demagnified when the substrate is thin compared to its width. This allows one to accurately control the width of the gap. Moreland & Ekin (115) first used the MCBJ to study electron tunneling, and Muller et al. (116) pioneered the technique to create metallic quantum point contacts and to demonstrate conductance quantization (29). Reed et al. (118) applied MCBJ to measure electron transport in molecules. They first formed a molecular-scale gap between two electrodes and then exposed the electrodes to a solution containing benzenedithiol. After evaporation of the solvent, they measured a finite current between the electrodes and attributed it to electron transport through the molecules. Kergueris et al. (119) measured electron transport through oligothiophenes terminated with dithiols using an MCBJ in a similar way. These pioneering works demonstrate the feasibility of studying electron transport in molecules using the MCBJ; however, how many molecules were involved in the conduction and whether the molecules were bound to both electrodes were not clear. Reichert et al. (120) provided strong evidence of single-molecule conductance measurements using an MCBJ setup. In their experiment, they first applied a droplet of a solvent containing dithiol molecules to the electrode pair formed with the MCBJ to allow the molecules to adsorb onto the electrodes. The gap was then closed while maintaining a fixed voltage across the electrodes, during which time they recorded the current (Figure 5c). Initially, the current grew exponentially, owing to electron tunneling across the gap. When the gap was decreased to a certain width, sometimes the current locked into a stable value weakly dependent on the gap width (121). At that point, the first molecule was presumably attached to the opposite side. The measured I-V (Figure 5d) curves were asymmetric for asymmetric molecules, which suggests a single or a small number of molecules bridging the gap. Smit et al. (122) reported that a hydrogen molecule forms a stable bridge between platinum electrodes in their quantum point contact measurements in the presence of H2 . The measured conductance determined from the hydrogen bridge is close to 1G0 , nearly perfect conductance or 100% transmission through H2 . They also studied the vibrations of the molecules using point contact spectroscopy (123). 3.3.2. Scanning-tunneling-microscopy-based break junction. Xu & Tao (124) developed an STM–break junction method that quickly creates thousands of

544

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

Figure 5 (a) Schematics of a microfabricated mechanically controllable break junction (MCBJ) (88). (b) Scanning-electron-microscope picture of the lithographically fabricated break junction (92). (c) Change of current recorded during the formation of a molecular junction. After a region of roughly exponential increase of the current, the current suddenly locks into a remarkably stable value (plateau), nearly independent of the distance. (d) I-V and dI/dV of a molecular junction. Images reprinted with permission from References 32, 117, 120, and 121.

molecular junctions by repeatedly moving a scanning-tunneling-microscope tip into and out of contact with the substrate electrode in the presence of sample molecules (Figure 6). The molecules in the study have two end groups that can bind to the tip and the substrate electrodes, respectively. The process can be divided into three steps: (a) A PZT drives the tip toward the substrate surface until it contacts the molecules bound to the substrate. During the contact period, one or more molecules may bind to the tip via the second end group. (b) The tip is pulled away from the substrate, and then the molecules break contact with one of the two electrodes individually, which


545

ARI

22 February 2007


ANRV308-PC58-20

11:34

Figure 6 (a) Measurement of single-molecule conductance using a scanning tunneling microscopy (STM) break junction. A scanning-tunneling-microscope tip is pushed into contact with molecules adsorbed on an electrode with a piezoelectric transducer (PZT) in a solution containing the sample molecules, during which the molecules have a chance to bridge the tip and substrate electrodes. The tip is then pulled away from the substrate to break the molecules from contacting the electrodes. (b) The current (conductance) for viologen dithiol (N, N -bis(6-thiohexyl)-4,4 -bipyridinium bromide) recorded during the pulling process shows stepwise features. Bias voltage = −0.1 V. These curves reflect the typical current features for thousands of molecular junctions repeatedly formed under the same conditions. (c) Conductance histogram constructed from the individual measurements reveals well-defined peaks near integer multiples of a fundamental value, ∼1 nS, which is identified as the conductance of a single molecule. Reprinted with permission from Reference 85.

Conductance histogram: a statistical distribution of conductance values of a large ensemble of molecular junctions with microscopically different contact geometries

546

is shown as a series of stepwise decreases in the current (Figure 6b). (c) The process is repeated until a large number of molecular junctions are created and measured. The conductance histogram of these molecular junctions exhibits peaks located near integer multiples of a fundamental value, which is used as a signature to identify a single-molecule conductance. During the approaching stage, one may control the tip to either make contact with the substrate to form a quantum point contact or avoid the formation of a point contact between the tip and the substrate.

Chen et al.

ARI

22 February 2007

11:34

Unlike most of the MCBJ experiments that record the approaching processes, this method focuses on the separation process. Because only those molecules bound to two electrodes can be stretched until breakdown during the separation process, this method selectively measures the conductance of those molecules bound to both electrodes. Another difference is that the creation of the molecular junctions and the measurement of the conductance are performed in an organic solvent or aqueous electrolyte, so one can easily introduce the sample molecules via the solution and control the electron transport through the molecules electrochemically. Finally, the scanning-tunneling-microscope tip images the substrate surface before performing the electron-transport measurement, so one can place the tip on an atomically flat area or move the tip laterally to a fresh area of the substrate during the measurement. This freedom comes at the cost of mechanical and thermal stability, however, which is typically not as good as the MCBJ. Different groups have used this or similar approaches to determine the conductance of many molecules (85, 125–127). Despite the success, several issues remain to be resolved and understood (see Section 4). 3.3.3. Conducting atomic-force-microscopy break junction. The stepwise change in the conductance curves shown in Figure 6b was attributed to the breakdown of individual molecules during the separation of the electrodes. Xu et al. (80) confirmed this by measuring the electromechanical properties of the molecular junctions with a conducting atomic force microscope (Figure 7). Each abrupt conductance decrease is accompanied by an abrupt decrease in the force, corresponding to the breakdown of a molecule from contacting the electrodes. Further pulling causes only a slight change in the conductance but an approximately linear increase in the force. When the force reaches a certain threshold, another molecule junction breaks

b

Conductance (10– 4 G0)

a

6

Conductance 8

4 Force 4 2

0

0

Stretching Figure 7 (a) Simultaneous conductance and force measurements of a molecular junction during breakdown using a conducting atomic force microscope setup. (b) As the individual molecules break, the conductance decreases in a stepwise fashion (red), and associated with each conductance step, the force (blue) also decreases abruptly (83).


547

Force (nN)


ANRV308-PC58-20

ANRV308-PC58-20

ARI

22 February 2007

11:34

down, resulting in additional abrupt decreases in the conductance and the force. The average breakdown force required to break Au-octanedithiol junctions is ∼1.5 nN (128), which is about the same as the force required to break an Au-Au bond under the same conditions, indicating the breakdown probably takes place at the Au-Au bond. The conducting AFM break junction method also allows one to study the electromechanical properties of single molecules.


3.3.4. Other scanning-tunneling-microscopy-controlled point contact measurements. The STM break junction method described above forms a molecular junction by first contacting the molecules with the tip and then pulling the tip away. Haiss et al. (129) developed an STM trapping method to bridge molecules between the tip and the substrate. The tip was brought close to but not into contact with the substrate covered with the dithiol molecules. They observed sudden jumps in the tunneling current, which they attributed to the spontaneous chemical-bond formation between the tip and the molecule. Yasuda et al. (130) showed recently that one can form a molecular junction by moving the tip toward a given molecule until a chemical bond formed between the tip and the molecule. They first imaged isolated target molecules [α,ω-bis(acetylthio)terthiophene and α,ω-bis-(acetylseleno)terthiophene] embedded in an octanemonothiol monolayer. The target molecules are terminated either with two thiol or selenium groups that can bridge the substrate and the tip, but the octanemonothiols are bound to the substrate via its sole thiol group. They first located the target molecules and then placed the tip above a molecule and moved it toward the chosen molecule. When the tip formed a chemical bond with the terminal group (S or Se) of the molecule, the conductance jumped to a much higher conductance.

4. CRITICAL ISSUES IN SINGLE-MOLECULE CONDUCTANCE MEASUREMENTS The mechanical methods, especially when combined with force measurements and statistical analysis, have the following distinctive features: (a) Peaks in the conductance histogram located at integer multiples of a fundamental value can be used as a signature to identify the conductance of a single molecule. (b) The simultaneously measured force tells us if a molecule is properly bound to two electrodes. (c) Statistical analysis reduces the chances of mistaking impurities as sample signals. (d ) Mechanical methods work in solution and in vacuum, which not only makes it easy to introduce various sample molecules into the measurement, but also provides a controlled and inert environment for the measurement. However, there are several issues that require further study.

4.1. Molecule-Electrode Contact Geometry In STM– or conducting AFM–break junction methods, a large number of molecular junctions are repeatedly created, but these molecular junctions differ in their contact geometries (Figure 8). In fact, the conductance of the last step, corresponding to

548

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

Figure 8 (a) Different contact geometries of a 1,6-hexanedithiol molecule bound to two Au electrodes. (b) Calculated conductance values of the molecule for the different contact geometries plotted versus the number of atoms protruding out of the crystal planes of the electrodes. (c) Distribution of conductance values with 1296 different contact geometries. Reprinted with permission from References 14 and 16.

the formation of a single-molecule junction, varies considerably from one junction to another, which reflects atomic-scale variations in the contact geometries (13, 14, 16, 17). Guo and colleagues (16) calculated the conductance values of molecules with hundreds of different contact geometries (Figure 8c). By assuming that the statistical weight of each geometry is equal, they obtained a distribution of the conductance


549

ANRV308-PC58-20

ARI

22 February 2007

HOMO: highest occupied molecular orbital


LUMO: lowest unoccupied molecular orbital

11:34

values. The distribution for 1,6-hexanedithiol bound to Au electrodes has a peak near ∼2 × 10−3 G0 , which is in good agreement with the experimental peak position at ∼1.2 × 10−3 G0 (124). In addition to various binding sites of the anchoring groups (e.g., S) on Au electrodes, Muller (14) considered the possibility that the anchoring groups may bind to one or more Au atoms protruding out of the electrode crystal planes (Figure 8a). For 1,6-hexanedithiol, most of the 16 different contact geometries have conductance values close to ∼2 × 10−3 G0 (Figure 8b), in close agreement with both the experiment and the calculations by Guo et al. (16). In the case discussed above, pronounced peaks appear in the conductance histogram, which are used to identify the conductance of single molecule. This method fails if the distribution is too broad, as shown by Ulrich et al. (131). We also found in our lab that some molecules, especially small conjugate molecules, exhibit welldefined steps in the individual conductance traces, but the conductance histograms display only broad distributions (132). Another possibility is that for (at least) some molecules, more than one set of conductance peaks appear in the histograms. Li et al. (133) studied the conductance of single N-alkanedithiols (N = 6, 8 and 10) covalently bound to Au electrodes. For each molecule, the conductance histogram reveals three sets of well-defined peaks near integer multiples of different conductance values. The two higher conductance sets can be described by the tunneling model with an identical decay constant of 0.84 A˚ −1 and interpreted in terms of different moleculeelectrode contact geometries (134). The lowest one is thermally activated, which was previously reported by Haiss et al. (23), and is attributed to the thermal fluctuation of the molecular conformation. The distribution of different contact geometries appears to depend on the way the molecular junctions are formed. For example, Wandlowski and colleagues (85) measured the conductance of dithiol molecules using an STM break junction by carefully avoiding the contact of the tip with the Au substrate. They found one set of pronounced peaks in the conductance histograms. Conversely, Venkataraman et al. (127) reported that diamine-terminated molecules gave well-defined peaks in the conductance histograms by forcing the two electrodes to form a point contact during the approaching stage. In their case, the formation of point contacts was believed to be crucial for the formation of an amine-Au bond.

4.2. Molecule-Electrode Energy-Level Alignment The conductance of a molecule depends on the alignment of the molecular energy levels, especially the highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO), relative to the Fermi levels of the electrodes. Typically, the Fermi level is positioned in the LUMO-HOMO gap of the molecule because if the HOMO or LUMO is close to the Fermi level of an electrode, electrons transfer between the molecule and the electrode, and consequently the molecules are oxidized or reduced spontaneously. The energy-level alignment is determined by the intrinsic properties of the molecule and the electrodes, and also by the interactions between the molecule and the two electrodes, which are often difficult to determine for both theory and experiment (135, 136).

550

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

Surface spectroscopy techniques, such as ultraviolet photoelectron spectroscopy and X-ray photoelectron spectroscopy, can provide useful information about the energy-level alignments of molecules adsorbed on surfaces (137). STM tunneling spectroscopy has also been used to study the electronic states of molecules adsorbed on surfaces. Hipps and colleagues combined the tunneling spectroscopy with ultraviolet photoelectron spectroscopy to study metallo-porphyrins and phthalocyanines on Au (138) and resolved different molecular energy levels and their relative positions to the Au Fermi level (77). In electrolytes, one may gain valuable insights into the energy-level alignment by measuring the oxidization or reduction potentials (Figure 9) (139). One may even tune the alignment by adjusting the potentials of the electrodes relative to a reference electrode inserted in the electrolyte. When the LUMO or HOMO is brought close to the Fermi levels, electrochemical oxidation or reduction occurs. For molecules that can be reversibly oxidized or reduced, significant changes in the conductance occur. This has lead to electrochemical gate control of the conductance of single redox molecules that is analogous to field effect transistors (82, 140–142). This behavior has been observed in several redox systems (82, 140–143). For example, Haiss et al. (140) reported an interesting potential-dependent conductance in viologen derivatives. Another example is perylene tetracarboxylic diimide terminated with thiol groups (Figure 9c) (141). The conductance of a single perylene tetracarboxylic diimide molecule covalently bound to two Au electrodes increases reversibly over nearly three orders of magnitude as one shifts the LUMO of the molecule toward the Fermi level of the electrodes.

4.3. Effect of External Force The mechanical methods can create a molecular junction by controlling the separation between two electrodes. It is natural to ask how the applied force affects the measured conductance of the molecules. In the individual conductance traces recorded during the breakdown of the molecular junctions, the step is rather flat, which means the force does not significantly affect the measured conductance until the contact breaks down. However, carefully examining the steps, one can see a finite decrease, on average, in the conductance upon stretching. In the case of alkanedithiols, the decrease is only 10%–20% before breakdown (81). Another observation is that the distance over which a molecular junction can be stretched is typically several angstroms. Clearly a small molecule cannot be stretched over such a large distance without tearing the molecule apart. The reason for such a large stretching distance is that most of the stretching actually takes place in the Au atoms near the molecule-electrode contact rather than in the molecule, itself. When the force reaches ∼1.5 nN, the contact breaks, and the maximum force that one applies to the molecule is ∼1.5 nN. For molecules such as alkanedithiols, such a force is not sufficient to change the electronic states of the molecules or distort the structure of the molecule significantly. However, for conjugated molecules, such as oligothiophenes, the force can cause a much greater change in the conductance, owing to a stronger coupling between the electronic states and the structures of the molecule than that for alkanedithiols (81).


551

ANRV308-PC58-20

ARI

22 February 2007

(a) A redox molecule is covalently linked to two electrodes in an electrochemical cell. (Counter electrode is not shown here for clarity.) (b) In terms of the energy diagram, the electrochemical gate voltage (Vg ) shifts the molecular energy levels up and down relative to the Fermi energy levels of the source and drain electrodes, and the current is driven by a finite bias voltage (Vsd ) between the two electrodes. (c) The current through a single perylene tetracarboxylic diimide molecule as a function of electrochemical gate voltage (Vg ).

a Reference electrode

Gate Working electrode 1

Vg

Source

Working electrode 2 O

O

N

N

O

O

Drain

Vsd

Redox molecule

b Molecular level Fermi level

c

Fermi level

10

Current (nA)


Figure 9

11:34

1

0.1

–0.8

–0.6

–0.4

–0.2

Vg (V vs. Ag/AgCl) One may also expect a stronger effect of the applied force on the conductance for softer molecules.

4.4. Effect of Environment The local environment (such as solvent molecules, ions, pH, and dopants) of a molecule can significantly affect the conductance of the molecule. Xiao et al. (144) measured conductance on several oligopeptides terminated with dithiols as a function of solution pH and observed the dependence of the conductance on pH (Figure 10).

552

Chen et al.

ANRV308-PC58-20

ARI

22 February 2007

11:34

a

b 4.8

HS

SH

N H O

NH 2

G(10–3G0)

O H N

4.4

Cysteamine-Gly-Cys


4.0 0

2

4

6

8

10

12

pH Figure 10 (a) Molecular structure of a peptide, Cysteamine-Gly-Cys. (b) Dependence of peptide conductance on the solution pH.

They traced the pH dependence to the protonation or deprotonation of NH2 and COOH in the peptides, which changes electronic states and local charge distributions of the molecules. The conductance of a molecule may also depend on the surrounding ions (6). For example, the presence of Cu2+ and Ni2+ in solution changes the conductance of cysteamine-Gly-Cys by ∼300 and ∼100 times, respectively, owing to strong binding of the metal ions to the molecule. In a recent study by STM in an ultrahigh vacuum environment, a charged dangling bond of Si induced a large change in the electronic states of a nearby adsorbed molecule, and the researchers attributed this observation to the electric field by the charge (92). In the case of peptides, the binding of Ni2+ and Cu2+ is strong and known to induce conformational changes in the peptides. Therefore, in addition to an electrostatic effect, conformational changes may be responsible for the change in conductance. For certain molecules, the dependence of conductance on local environment may be traced to the change in doping. Chen et al. (145) measured the conductance of a hepta-aniline oligomer attached to Au electrodes in toluene and in electrolyte. The single-molecule current-voltage characteristic is linear in toluene but displays negative differential resistance in an acidic electrolyte. The conductivity of bulk polyaniline increases dramatically when it is oxidized in an acidic medium, in which acid plays the role of doping the polymer materials. Similar to the bulk polyaniline, the conductance of a single aniline oligomer also increases in the oxidized form in acid. These results show the sensitive dependence of the conductance of the redox molecule on the solvent, and on the doping condition.

Local heating: heating effect in a molecular junction owing to an electrical current passing through the junction

4.5. Current-Induced Local-Heating Effects Local heating is known to be a critical factor in the design of conventional siliconbased microelectronics. When current passes through a single molecule, how hot it gets is an important question. In a nanoscale junction, the inelastic electron mean free


553

14

ARI

22 February 2007

11:34

path is often large compared with the size of the junction, so each electron is expected to release only a small amount of its energy to phonons (heat) during transport in the junction (66, 67). However, substantial local heating may still arise owing to the large current density and thus power per atom. Recently, researchers have reported experimental studies of local-heating effects in quantum point contacts (146, 147). In the case of molecules, the current density is much lower, but the thermal conduction of molecules may also be poorer than that of the metal contacts. Theoretical calculations have found a finite increase in the temperature of the molecules as a result of the inelastic scattering of electrons (148, 149). Huang et al. (150) reported on an experimental approach to determine the local temperature in a single molecule covalently bound to two Au electrodes. They measured the force required to break the attachment of the molecules to the electrodes. Because the breakdown process is thermally activated, the average breakdown force is sensitive to the local temperature of the molecule-electrode contact and can be used to estimate the effective local temperature of the molecular junction. For octanedithiol, the temperature with a bias voltage of 1 V (current ∼20 nA) increases by ∼25 K above room temperature.


ANRV308-PC58-20

5. OUTLOOK The conductance of a single molecule depends not only on the intrinsic properties of the molecule and the probing electrodes, but also on the molecule-electrode contacts and its local environment. To determine the conductance unambiguously, one wishes to fabricate a molecular junction with molecule-electrode contacts that are well defined on the atomic scale and carry out the measurement in a controlled environment. Fabricating such contacts reproducibly has been a challenge. Despite the difficulty, researchers have made solid advances recently. For example, STM has been used to create a well-defined metal-molecule-molecule junction by precisely placing individual metal atoms into contact with a molecule adsorbed on a substrate. To measure the conductance, the remaining challenges are to wire the two metal ends to an external circuit and to ensure that the electronic coupling between the molecular junction and the underlying substrate is negligible. Another important development is various break junction techniques. By combining a mechanical break junction with simultaneous force and conductance measurements, one can determine if the measured conductance is a result of a single molecule and how the molecule is bound to the probing electrodes. The approach also allows one to create a large ensemble of molecular junctions with microscopically different molecule-electrode contacts and to perform statistical analysis on the conductance values of the molecular junctions. A peak in the distribution gives an averaged conductance of the molecule. The approach is simple and has been used to measure the electron-transport properties of many different molecules in vacuum and in solutions. However, the exact molecule-electrode contact geometries created by the break junction methods are usually unknown, and techniques that can provide atomic-scale structural information of the molecule-electrode contact will be of great importance to the field.

554

Chen et al.

ANRV308-PC58-20

ARI

22 February 2007

11:34

SUMMARY POINTS 1. Determining the conductance of single molecule is a basic task in molecular electronics. 2. To measure the conductance of a molecule, one must provide a reliable and reproducible contact between the molecule and two probing electrodes.


3. Measurement of single-molecule conductance has been difficult owing to the lack of a technique that can provide reliable and well-defined moleculeelectrode contacts. 4. STM-based methods can place metal atoms into contact with a single molecule on a substrate with atomic precision. Remaining challenges are to connect the metal atoms to the external electrodes for conductance measurements and to ensure the electronic coupling between the molecule and the substrate does not interfere with the conductance measurement. 5. Break junction methods can create a large number of molecular junctions with different contact geometries. Although these contact geometries are not known precisely, the methods allow one to perform statistical analysis and to determine an average conductance of a molecule. 6. The conductance of a molecule depends also on its local environment. 7. By carrying out the measurement in an electrochemical environment, one can tune the alignment of the molecular energy levels relative to the Fermi energy levels of the electrodes and thus control the conductance of the molecule. 8. Future techniques that can fabricate molecular junctions with moleculeelectrode contacts that are well defined on the atomic scale and that can characterize the atomic-scale structures of the molecule-electrode contacts will contribute enormously to the field of molecular electronics.

ACKNOWLEDGMENTS We thank NSF, DOE, VW, and DARPA (via AFOSR) for financial support.

LITERATURE CITED 1. Aviram A, Ratner M. 1974. Molecular rectifiers. Chem. Phys. Lett. 29:277– 83 2. Goldhaber-Gordon D, Montemerlo MS, Love JC, Opiteck GJ, Ellenbogen JC. 1997. Overview of nanoelectronic devices. Proc. IEEE 85:521–40 3. Kuznetsov AM, Ulstrup J. 1999. Electron Transfer in Chemistry and Biology: An Introduction to the Theory. Chichester: Wiley 4. Hush NS. 2003. An overview of the first half-century of molecular electronics. Ann. NY Acad. Sci. 1006:1–20


1. Presents the first proposal of a molecular diode device based on a donor-sigma bridge-acceptor structure, and is an inspiration for many recent works.

2. Discusses molecular electronics as one of the approaches toward nanometer-scale electronic-switching devices.

555

ARI

22 February 2007


ANRV308-PC58-20

22. Presents a tutorial review of fundamental aspects of electron transport through molecules and different approaches to compute molecular conduction properties.

556

11:34

5. Hihath J, Xu B, Zhang P, Tao N. 2005. Study of single-nucleotide polymorphisms by means of electrical conductance measurements. Proc. Natl. Acad. Sci. USA 102:16979–83 6. Xiao X, Xu B, Tao N. 2004. Changes in the conductance of single peptide molecules upon metal-ion binding. Angew. Chem. Int. Ed. Engl. 43:6148–52 7. Cui XD, Primak A, Zarate X, Tomfohr J, Sankey OF, et al. 2001. Reproducible measurement of single-molecule conductivity. Science 294:571–74 8. Magoga M, Joachim C. 1997. Conductance and transparence of long molecular wires. Phys. Rev. B. 56:4722–29 9. Yaliraki SN, Kemp M, Ratner MA. 1999. Conductance of molecular wires: influence of molecule-electrode binding. J. Am. Chem. Soc. 121:3428–34 10. Moresco F, Gross L, Alemani M, Rieder KH, Tang H, et al. 2003. Probing the different stages in contacting a single molecular wire. Phys. Rev. Lett. 91:036601 11. McCreery RL, Viswanathan U, Kalakodium RP, Nowak AM. 2006. Carbon/ molecule/metal molecular electronics junctions: the importance of “contacts”. Faraday Discuss. 131:33–43 12. Xue Y, Ratner MA. 2003. Microscopic study of electrical transport through individual molecules with metallic contacts. II. Effect of the interface structure. Phys. Rev. B. 68:115406 13. Ke SH, Baranger HU, Yang WT. 2004. Molecular conductance: chemical trends of anchoring groups. J. Am. Chem. Soc. 126:15897–904 14. Muller KH. 2006. Effect of the atomic configuration of gold electrodes on the electrical conduction of alkanedithiol molecules. Phys. Rev. B. 73:045403 15. Beebe JM, Engelkes VB, Miller LL, Frisbie CD. 2002. Contact resistance in metal-molecule-metal junctions based on aliphatic sams: effects of surface linker and metal work function. J. Am. Chem. Soc. 124:11268–69 16. Hu YB, Zhu Y, Gao HJ, Guo H. 2005. Conductance of an ensemble of molecular wires: a statistical analysis. Phys. Rev. Lett. 95:156803 17. Basch H, Cohen R, Ratner MA. 2005. Interface geometry and molecular junction conductance: geometric fluctuation and stochastic switching. Nano Lett. 5:1668–75 18. Schmickler W, Widrig C. 1992. The investigation of redox reactions with a scanning tunneling microscope. J. Electroanal. Chem. 336:213–21 19. Kuznetsov AM, Sommer-Larsen P, Ulstrup J. 1992. Resonance and environmental fluctuation effects in STM currents through large adsorbed molecules. Surf. Sci. 275:52–64 20. Weiss EA, Tauber MJ, Kelley RF, Ahrens MJ, Ratner M, Wasielewski MR. 2005. Conformationally gated switching between superexchange and hopping within oligo-p-phenylene-based molecular wires. J. Am. Chem. Soc. 127:11842–50 21. Nitzan A. 2001. Electron transmission through molecules and molecular interfaces. Annu. Rev. Phys. Chem. 52:681–750 22. Selzer Y, Cabassi MA, Mayer TS, Allara DL. 2004. Thermally activated conduction in molecular junctions. J. Am. Chem. Soc. 126:4052–53 23. Haiss W, Van Zalinge H, Bethell D, Ulstrup J, Schiffrin DJ, Nichols RJ. 2006. Thermal gating of the single molecule conductance of alkanedithiols. Faraday Discuss. 131:253–64

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

24. Di Ventra M, Kim SG, Pantelides ST, Lang ND. 2001. Temperature effects on the transport properties of molecules. Phys. Rev. Lett. 86:288–91 25. Salomon A, Cahen D, Lindsay S, Tomfohr J, Engelkes VB, Frisbie CD. 2003. Comparison of electronic transport measurements on organic molecules. Adv. Mater. 15:1881–90 26. Tomfohr J, Ramachandran G, Sankey OF, Lindsay SM. 2005. Making Contacts to Single Molecules: Are We Nearly There Yet? Berlin: Springer 27. Gimzewski JK, Moller R. 1987. Transition from the tunneling regime to point contact studied using scanning tunneling microscopy. Phys. Rev. B 36:1284–87 28. Pascual JI, Mendez J, Gomez-Herrero J, Baro AM, Garcia N, Binh VT. 1993. Quantum contact in gold nanostructures by scanning tunneling microscopy. Phys. Rev. Lett. 71:1852–55 29. Krans JM, van Ruitenbeek JM, Fisun VV, Yanson IK, de Jongh LJ. 1995. The signature of condutance quantization in metallic point contacts. Nature 375:767–69 30. Agrait N, Rodrigo JG, Vieira S. 1993. Conductance steps and quantization in atomic-size contacts. Phys. Rev. B 52:12345–48 31. Yanson AI, Rubio Bollinger G, van den Brom HE, Agrait N, van Ruitenbeek JM. 1998. Formation and manipulation of a metallic wire of single gold atoms. Nature 395:783–85 32. Agrait N, Yeyati AL, van Ruitenbeek JM. 2003. Quantum properties of atomic-sized conductors. Phys. Rep. 377:81–279 33. Joachim C, Gimzewski JK, Aviram A. 2000. Electronics using hybrid-molecular and mono-molecular devices. Nature 408:541–48 34. Nitzan A, Ratner MA. 2003. Electron transport in molecular wire junctions. Science 300:1384–89 35. Carroll Rl, Gorman CB. 2002. The genesis of molecular electronics. Angew. Chem. Int. Ed. Engl. 41:4379–99 36. McCreery RL. 2004. Molecular electronic junctions. Chem. Mater. 16:4477–96 37. Seideman T, Guo H. 2003. Quantum transport and current-triggered dynamics in molecular tunnel junctions. J. Theor. Comp. Chem. 2:439–58 38. Xue Y, Ratner M. 2005. Theoretical principles of single-molecule electronics: a chemical and mesoscopic view. Int. J. Quantum Chem. 102:911–24 39. Selzer Y, Allara DL. 2006. Single-molecule electrical junctions. Annu. Rev. Phys. Chem. 57:593–623 40. Mantooth BA, Weiss PS. 2003. Fabrication, assembly, and characterization of molecular electronic components. Proc. IEEE 91:1785–802 41. Wang WY, Lee TH, Reed MA. 2005. Electronic transport in molecular self-assembled monolayer devices. Proc. IEEE 93:1815–24 42. Metzger RM. 2003. Unimolecular electrical rectifiers. Chem. Rev. 103:3803–34 43. Opila RL, Eng J. 2002. Thin films and interfaces in microelectronics: composition and chemistry as function of depth. Prog. Surf. Sci. 69:125–63 44. Martin AS, Sambles JR, Ashwell GJ. 1993. Molecular rectifier. Phys. Rev. Lett. 70:218–21


25. Compares experimental results on low-bias, room-temperature conductance of saturated alkane chains and conjugate molecules obtained in different electrode-moleculeelectrode test beds.

32. Presents a comprehensive review of quantum point contact, mechanically controlled break junctions, and relevant physical concepts and techniques.

39. Reviews single-molecule electrical junctions with emphasis on the application of single-electron transistor theories such as the Kondo effect and Zeeman splitting.

40. Summarizes many different experimental techniques with a focus on the self-assembled monolayer.

41. Presents a comprehensive study of the conduction mechanism of alkane films sandwiched between two Au electrodes.

557

ARI

22 February 2007

11:34

45. Fisher GL, Walker AV, Hooper AE, Tighe TB, Bahnck KB, et al. 2002. Bond insertion, complexation, and penetration pathways of vapor-deposited aluminum atoms with HO- and CH3 O-terminated organic monolayers. J. Am. Chem. Soc. 124:5528–41 46. Akkerman HB, Blom PWM, de Leeuw DM, de Boer B. 2006. Towards molecular electronics with large-area molecular junctions. Nature 441:69–72 47. Loo Y-L, Lang DV, Rogers JA, Hsu JWP. 2003. Electrical contacts to molecular layers by nanotransfer printing. Nano Lett. 3:913–17 48. Ojima K, Otsuka Y, Matsumoto T, Kawai T, Nakamatsu K, Matsui S. 2005. Printing electrode for top-contact molecular junction. Appl. Phys. Lett. 87:234110 49. Deng W, Yang L, Fujita D, Nejoh H, Bai C. 2000. Silver ions adsorbed to selfassembled monolayers of alkanedithiols on gold surfaces form Ag-dithiol-Au multilayer structures. Appl. Phys. A 71:639–42 50. Qu D, Uosaki K. 2006. Platinum layer formation on a self-assembled monolayer by electrochemical deposition. Chem. Lett. 35:258–59 51. Epple M, Bittner AM, Kuhnke A, Kern K, Zheng WQ, Tadjeddine A. 2002. Alkanethiolate reorientation during metal electrodeposition. Langmuir 18:773– 84 52. Manolova M, Ivanova V, Kolb D, Boyen H, Ziemann P, et al. 2005. Metal deposition onto thiol-covered gold: platinum on a 4-mercaptopyridine SAM. Surf. Sci. 590:146–53 53. Slowinski K, Chamberlain RV, Miller CJ, Majda M. 1997. Through-bond and chain-to-chain coupling two pathways in electron tunneling through liquid alkanethiol monolayers on mercury electrodes. J. Am. Chem. Soc. 119:11910–19 54. Tran E, Rampi MA, Whitesides GM. 2004. Electron transfer in a HgSAM//SAM-Hg junction mediated by redox centers. Angew. Chem. Int. Ed. Engl. 43:3835–39 55. Ranganathan S, Steidel I, Anariba F, McCreery RL. 2001. Covalently bonded organic monolayers on a carbon substrate: a new paradigm for molecular electronics. Nano Lett. 1:491–94 56. Salomon A, Arad-Yellin R, Shanzer A, Karton A, Cahen D. 2004. Stable roomtemperature molecular negative differential resistance based on moleculeelectrode interface chemistry. J. Am. Chem. Soc. 126:11648–57 57. Liu Y, Yu H. 2003. Molecular passivation of classical mercury-silicon (p-type) diode junctions: alkylation, oxidation, and alkylsilation. J. Phys. Chem. B 107:7803–11 58. Wold DJ, Frisbie CD. 2000. Formation of metal-molecule-metal tunnel junctions: microcontacts to alkanethiol monolayers with a conducting AFM tip. J. Am. Chem. Soc. 122:2970–71 59. Fan FRF, Yang JP, Cai LT, Price DW, Dirk SM, et al. 2002. Charge transport through self-assembled monolayers of compounds of interest in molecular electronics. J. Am. Chem. Soc. 124:5550–60 60. Kushmerick JG, Holt DB, Pollack SK, Ratner MA, Yang JC, et al. 2002. Effect of bond-length alternation in molecular wires. J. Am. Chem. Soc. 124:10654–55


ANRV308-PC58-20

558

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

61. Stewart DR, Ohlberg DAA, Beck PA, Chen Y, Williams RS, et al. 2004. Molecule-independent electrical switching in Pt/organic monolayer/Ti devices. Nano Lett. 4:133–36 62. Collier CP, Mattersteig G, Wong EW, Luo Y, Beverly K, et al. 2000. A [2]catenane-based solid state electronically reconfigurable switch. Science 289:1172–75 63. Chen J, Reed MA, Rawlett AM, Tour JM. 1999. Large on-off ratios and negative differential resistance in a molecular electronic device. Science 286:1550–52 64. Petta JR, Slater SK, Ralph DC. 2004. Spin-dependent transport in molecular tunnel junctions. Phys. Rev. Lett. 93:136601 65. Mbindyo JKN, Mallouk TE, Mattzela JB, Kratochvilova, Irena R, et al. 2002. Template synthesis of metal nanowires containing monolayer molecular junctions. J. Am. Chem. Soc. 124:4020–26 66. Wang W, Lee T, Reed MA. 2004. Elastic and inelastic electron tunneling in alkane self-assembled monolayers. J. Phys. Chem. B. 108:18398–407 67. Kushmerick JG, Lazorcik J, Patterson CH, Shashidhar R, Seferos DS, Bazan GC. 2004. Vibronic contributions to charge transport across molecular junctions. Nano Lett. 4:639–42 68. Chabinyc ML, Chen X, Holmlin RE, Jacobs H, Skulason H, et al. 2002. Molecular rectification in a metal-insulator-metal junction based on self-assembled monolayers. J. Am. Chem. Soc. 124:11730–36 69. Jiang P, Morales GM, You W, Yu L. 2004. Synthesis of diode molecules and their sequential assembly to control electron transport. Angew. Chem. Int. Ed. Engl. 43:4471–75 70. Metzger RM. 1999. Electrical rectification by a molecule: the advent of unimolecular electronic devices. Acc. Chem. Res. 32:950–57 71. Blum AS, Kushmerick JG, Long DP, Patterson CH, Yang JC, et al. 2005. Molecularly inherent voltage-controlled conductance switching. Nat. Mater. 4:167–72 72. Chen J, Wang W, Klemic MA, Reed MA, Axelrod BW, et al. 2002. Molecular wires, switches and memories. Ann. NY Acad. Sci. 960:69–99 73. Kiehl RA, Le JD, Candra P, Hoye RC, Hoye TR. 2006. Charge storage model for hysteretic negative-differential resistance in metal-molecule-metal junctions. Appl. Phys. Lett. 88:172102–4 74. Slzer Y, Cai LT, Cabassi MA, Yao YX, Tour JM, et al. 2005. Effect of local environment on molecular conduction: single molecule versus self-assembled monolayer. Nano Lett. 5:61–65 75. Gimzewski JK, Joachim C. 1999. Nanoscale science of single molecules using local probes. Science 283:1683–88 76. Stipe BC, Rezaei MA, Ho W. 1998. Inducing and viewing the rotation motion of a single molecule. Science 279:1907–9 77. Mazur U, Hipps KW. 1999. Orbital-mediated tunneling, inelastic electron tunneling, and electrochemical potentials for metal phthalocyanine thin films. J. Phys. Chem. B. 103:9721–27


559


ANRV308-PC58-20

ARI

22 February 2007

80. Presents simultaneous conductance and force measurements showing that the stepwise conductance change in the break junction is a result of individual molecules.

560

11:34

78. Datta S, Tian WD, Hong SH, Reifenberger R, Henderson JI, Kubiak CP. 1997. Current-voltage characteristics of self-assembled monolayers by scanning tunneling microscopy. Phys. Rev. Lett. 79:2530–33 79. Moresco F. 2004. Manipulation of large molecules by low-temperature STM: model systems for molecular electronics. Phys. Rep. 399:175–25 80. Xu BQ, Xiao XY, Tao NJ. 2003. Measurement of single molecule electromechanical properties. J. Am. Chem. Soc. 125:16164–65 81. Xu B, Li X, Xiao X, Sakaguchi H, Tao N. 2005. Electromechanical and conductance switching properties of single oligothiophene molecules. Nano Lett. 5:1491–95 82. Tao NJ. 1996. Probing potential-tuned resonant tunneling through redox molecules with scanning tunneling mircoscopy. Phys. Rev. Lett. 76:4066–69 83. Albrecht T, Guckian A, Ulstrup J, Vos JG. 2005. Transistor-like behavior of transition metal complexes. Nano Lett. 5:1451–55 84. Gittins DI, Bethell D, Schiffrin DJ, Nichols RJ. 2000. A nanometer-scale electronic switch consisting of a metal cluster and redox-addressable groups. Nature 408:67–69 85. Li Z, Han B, Meszaros G, Pobelov I, Wandlowski T, et al. 2006. Twodimensional assembly and local redox-activity of molecular hybrid structures in an electrochemical environment. Faraday Discuss. 131:121–43 86. Joachim C, Gimzewski JK. 1997. An electromechanical amplifier using a single molecule. Chem. Phys. Lett. 265:353–57 87. Zeng C, Wang H, Wang B, Yang J, Hou JG. 2000. Negative differentialresistance device involving two C60 molecules. Appl. Phys. Lett. 77:3595–97 88. Guisinger NP, Greene ME, Basu R, Baluch AS, Hersam MC. 2004. Room temperature negative differential resistance through individual organic molecules on silicon surfaces. Nano Lett. 4:55–59 89. Wassel RA, Credo GM, Fuierer RR, Feldheim DL, Gorman CB. 2004. Attenuating negative differential resistance in an electroactive self-assembled monolayer-based junction. J. Am. Chem. Soc. 126:295–300 90. Bumm LA., Arnold JJ, Cygan MT, Dunbar TD, Burgin TP, et al. 1996. Are single molecular wires conducting? Science 271:1705–7 91. Qiu XH, Nazin GV, Ho W. 2004. Mechanisms of reversible conformational transitions in a single molecule. Phys. Rev. Lett. 93:196806 92. Piva PG, DiLabio GA, Pitters JL, Zikovsky J, Rezeq M, et al. 2005. Field regulation of single-molecule conductivity by a charged surface atom. Nature 435:658–61 93. Langlais VJ, Schlittler RR, Tang H, Gourdon A, Joachim C, Gimzewski JK. 1999. Spatially resolved tunneling along a molecular wire. Phys. Rev. Lett. 83:2809–12 94. Andres RP, Bein T, Dorogi M, Feng S, Henderson JI, et al. 1996. “Coulomb staircase” at room temperature in a self-assembled molecular nanostructure. Science 272:1323–25 95. Ramachandran GK, Tomfohr JK, Li J, Sankey OF, Zarate X, et al. 2003. Electron transport properties of a carotene molecule in a metal-(single molecule)metal junction. J. Phys. Chem. 107:6162–69

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

96. Samanta MP, Tian W, Datta S, Henderson JI, Kubiak CP. 1996. Electronic conduction through organic molecules. Phys. Rev. B. 53:R7626–29 97. Nazin GV, Qiu XH, Ho W. 2003. Visualization and spectroscopy of a metalmolecule-metal bridge. Science 302:77–81 98. Repp J, Meyer G, Paavilainen S, Olsson FE, Persson M. 2006. Imaging bond formation between a gold atom and pentacene on an insulating surface. Science 312:1196–99 99. Park J, Pasupathy AN, Goldsmith JL, Chang C, Yaish Y, et al. 2002. Coulomb blockade and the Kondo effect in single-atom transistors. Nature 417:722–25 100. Liang WJ, Shores M, Bockrath M, Long JR, Park H. 2002. Kondo resonance in a single-molecule transistor. Nature 417:725–29 101. Li CZ, Bogozi A, Huang W, Tao NJ. 1999. Fabrication of stable metallic nanowires with quantized conductance. Nanotechnology 10:221–23 102. Morpurgo AF, Marcus CM, Robinson DB. 1999. Controlled fabrication of metallic electrodes with atomic separation. Appl. Phys. Lett. 14:2084–86 103. Kervennic YV, Thijssen JM, Vanmaekelbergh D, Dabirian R, Jenneskens LW, et al. 2006. Charge transport in three-terminal molecular junctions incorporating sulfur-end-functionalized tercyclohexylidene spacers. Angew. Chem. Int. Ed. Engl. 45:2540–42 104. Li XL, He HX, Xu BQ, Xiao X, Tsui R, et al. 2004. Electron transport properties of electrochemically and mechanically formed molecular junctions. Surf. Sci. 573:1–10 105. Lee J-O, Lientschnig G, Wiertz F, Struijk M, Janssen RAJ, et al. 2003. Absence of strong gate effects in electrical measurements on phenylene-based conjugated molecules. Nano Lett. 3:113–17 106. Luber SM, Strobel S, Tranitz HP, Wegscheider W, Schuh D, Tornow M. 2005. Nanometre spaced electrodes on a cleaved AlGaAs surface. Nanotechnology 16:1182–85 107. Kubatkin S, Danilov A, Hjort M, Cornil J, Bredas J-L, et al. 2003. Singleelectron transistor of a single organic molecule with access to several redox states. Nature 425:698–701 108. Zhitenev NB, Meng H, Bao Z. 2002. Conductance of small molecular junctions. Phys. Rev. Lett. 92:186805 109. Ghosh S, Halimun H, Mahapatro AK, Choi J, Lodha S, Janes D. 2005. Device structure for electronic transport through individual molecules using nanoelectrodes. Appl. Phys. Lett. 87:233509 110. Amlani I, Rawlett AM, Nagahara LA, Tsui RK. 2002. An approach to transport measurements of electronic molecules. Appl. Phys. Lett. 80:2761–63 111. Dadosh T, Gordin Y, Krahne R, Khivrich I, Mahalu D, et al. 2005. Measurement of the conductance of single conjugated molecules. Nature 436:677–80 112. Chae DH, Berry JF, Jung S, Cotton FA, Murillo CA, Yao Z. 2006. Vibrational excitations in single trimetal-molecule transistors. Nano Lett. 6:165–68 113. Yu LH, Keane ZK, Ciszek JW, Cheng L, Tour JM, et al. 2005. Kondo resonances and anomalous gate dependence in the electrical conductivity of single-molecule transistors. Phys. Rev. Lett. 95:256803


561

ARI

22 February 2007

11:34

114. de Picciotto A, Klare JE, Nuckolls C, Baldwin K, Erbe A, Willett R. 2005. Prevalence of Coulomb blockade in electro-migrated junctions with conjugated and nonconjugated molecules. Nanotechnology 16:3110–14 115. Moreland J, Ekin JW. 1985. Electron tunneling experiments using Nb-Sn break junctions. J. Appl. Phys. 58:3888–95 116. Muller CJ, van Ruitenbeek JM, de Jongh LJ. 1992. Conductance and supercurrent discontinuities in atomic-scale metallic constrictions of variable width. Phys. Rev. Lett. 69:140–43 117. Ruitenbeek JMV, Allvarez A, Pineyro I, Grahmann C, Joyez P, et al. 1996. Adjustable nanofabricated atomic size contacts. Rev. Sci. Instrum. 67:108–11 118. Reed MA, Zhou C, Muller CJ, Burgin TP, Tour JM. 1997. Conductance of a molecular junction. Science 278:252–54 119. Kergueris C, Bourgoin JP, Palacin S, Esteve D, Urbina C, et al. 1999. Electron transport through a metal-molecule-metal junction. Phys. Rev. B. 59:12505–13 ¨ 120. Reichert J, Ochs R, Beckmann D, Weber HB, Mayor M, Lohneysen HV. 2002. Driving current through single organic molecules. Phys. Rev. Lett. 88:176804 121. Weber HB, Reichert J, Weigend F, Ochs R, Beckmann D, et al. 2002. Electronic transport through single conjugated molecules. Chem. Phys. 281:113–25 122. Smit RHM, Noat Y, Untiedt C, Lang ND, van Hemert MC, van Ruitenbeek JM. 2002. Measurement of the conductance of a hydrogen molecule. Nature 419:906–9 123. Djukic D, Thygesen KS, Untiedt C, Smit RHM, Jacobsen KW, van Ruitenbeek JM. 2005. Stretching dependence of the vibration modes of a single-molecule Pt-H2 -Pt bridge. Phys. Rev. B 71:161402 124. Xu BQ, Tao NJ. 2003. Measurement of single molecule conductance by repeated formation of molecular junctions. Science 301:1221–23 125. He J, Chen F, Li J, Sankey OF, Terazono Y, et al. 2005. Electronic decay constant of carotenoid polyenes from single-molecule measurements. J. Am. Chem. Soc. 127:1384–85 126. Ishizuka K, Suzuki M, Fuju S, Takayama Y, Sato F, Fujihira M. 2006. Effect of molecule-electrode contacts on single-molecule conductivity of μ-conjugated system measured by scanning tunneling microscopy under ultrahigh vacuum. Jpn. J. Appl. Phys. 45:2037–40 127. Venkataraman L, Klare JE, Tam IW, Nuckolls C, Hybertsen MS, Steigerwald ML. 2006. Single-molecule circuits with well-defined molecular conductance. Nano Lett. 6:458–62 128. Rubio G, Agrait N, Vieira S. 1996. Atomic-sized metallic contacts: mechanical properties and electronic transport. Phys. Rev. Lett. 76:2302–5 129. Haiss W, Nichols RJ, van Zalinge H, Higgins SJ, Bethell D, Schiffrin DJ. 2004. Measurement of single molecule conductivity using the spontaneous formation of molecular wires. Phys. Chem. Chem. Phys. 6:4330–37 130. Yasuda S, Yoshida S, Sasaki J, Okutsu Y, Nakamura T, et al. 2006. Bond fluctuation of S/Se anchoring observed in single-molecule conductance measurements using the point contact method with scanning tunneling microscopy. J. Am. Chem. Soc. 128:7746–47


ANRV308-PC58-20

562

Chen et al.


ANRV308-PC58-20

ARI

22 February 2007

11:34

131. Ulrich J, Esrail D, Pontius W, Venkataraman L, Millar D, Doerrer LH. 2006. Variability of conductance in molecular junctions. J. Phys. Chem. B 110:2462–66 132. Xiao X, Brune D, He J, Lindsay SM, Gorman CB, Tao NJ. 2006. Redoxgated electron transport in electrically wired ferrocene molecules. Chem. Phys. 326:138–43 133. Li X, He J, Hihath J, Xu B, Lindsay SM, Tao N. 2006. Conductance of single alkanedithiols: conduction mechanism and effect of moleculeelectrode contacts. J. Am. Chem. Soc. 128:2135–41 134. He J, Sankey OF, Meyong L, Li XL, Tao NJ, Lindsay SM. 2006. Measuring single molecule conductance with break junctions. Faraday Discuss. 31:145–54 135. Xue Y, Datta S, Ratner MA. 2001. Charge transfer and “band lineup” in molecular electronic devices: a chemical and numerical interpretation. J. Chem. Phys. 115:4292–99 136. Heimel G, Romaner L, Brédas J-L, Zojer E. 2006. Interface energetics and level alignment at covalent metal-molecule junctions: conjugated thiols on gold. Phys. Rev. Lett. 96:196806 137. Kim B, Beebe JM, Jun Y, Zhu XY, Frisbie CD. 2006. Correlation between HOMO alignment and contact resistance in molecular junctions: aromatic thiols versus aromatic isocyanides. J. Am. Chem. Soc. 128:4970–71 138. Scudiero L, Barlow DE, Mazur U, Hipps KW. 2001. Scanning tunneling microscopy, orbital-mediated tunneling spectroscopy, and UV photoelectron spectroscopy of metal(II) tetraphenylporphyrins deposited from vapor. J. Am. Chem. Soc. 123:4073–80 139. Fan FRF, Lai RY, Cornil J, Karzazi Y, Bredas JL, et al. 2004. Electrons are transported through phenylene-ethynylene oligomer monolayers via localized molecular orbitals. J. Am. Chem. Soc. 126:2568–73 140. Haiss W, Van Zalinge H, Higgins SJ, Bethell D, Hoebenreich H, et al. 2003. Redox state dependence of single molecule conductivity. J. Am. Chem. Soc. 125:15294–95 141. Xu B, Xiao X, Yang X, Zang L, Tao N. 2005. Large gate modulation in the current of a room temperature single molecule transistor. J. Am. Chem. Soc. 127:2386–87 142. Chi Q, Farver Q, Ulstrup J. 2005. Long-range protein electron transfer observed at the single-molecule level: in situ mapping of redox-gated tunneling resonance. Proc. Natl. Acad. Sci. USA 102:16203–8 143. Alessandrini A, Salerno M, Frabboni S, Facci P. 2005. Single-metalloprotein wet biotransistor. App. Phys. Lett. 86:133902 144. Xiao X, Xu B, Tao N. 2004. Conductance titration of single-peptide molecules. J. Am. Chem. Soc. 126:5370–71 145. Chen F, He J, Nuckolls C, Roberts T, Klare JE, Lindsay S. 2005. A molecular switch based on potential-induced changes of oxidation state. Nano Lett. 5:503–6 146. Smit RHM, Untiedt C, van Ruitenbeek JM. 2004. The high-bias stability of monatomic chains. Nanotechnology 15:S472–78 147. Tsutsui M, Taninouchi Y, Kurokawa S, Sakai A. 2005. Effective temperature of Au nanocontacts under high biases. Jpn. J. Appl. Phys. 44:5188–90


133. Describes in detail an STM–break junction technique for single-molecule conductance measurements, including experimental setup and statistical analysis.

563

ANRV308-PC58-20

ARI

22 February 2007

11:34


148. Segal D, Nitzan A. 2002. Heating in current carrying molecular junctions. J. Chem. Phys. 117:3915–27 149. Chen Y-C, Zwolak M, Di Ventra M. 2003. Local heating in nanoscale conductors. Nano Lett. 3:1691–94 150. Huang ZF, Xu BQ, Chen YC, Di Ventra M, Tao NJ. 2006. Measurement of current-induced local heating in a single molecule junction. Nano Lett. 6:1240– 44

564

Chen et al.

AR308-FM

ARI

23 February 2007

13:44

Annual Review of Physical Chemistry


Contents

Volume 58, 2007

Frontispiece C. Bradley Moore p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p xvi A Spectroscopist’s View of Energy States, Energy Transfers, and Chemical Reactions C. Bradley Moore p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 1 Stochastic Simulation of Chemical Kinetics Daniel T. Gillespie p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 35 Protein-Folding Dynamics: Overview of Molecular Simulation Techniques Harold A. Scheraga, Mey Khalili, and Adam Liwo p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 57 Density-Functional Theory for Complex Fluids Jianzhong Wu and Zhidong Li p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p 85 Phosphorylation Energy Hypothesis: Open Chemical Systems and Their Biological Functions Hong Qian p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p113 Theoretical Studies of Photoinduced Electron Transfer in Dye-Sensitized TiO2 Walter R. Duncan and Oleg V. Prezhdo p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p143 Nanoscale Fracture Mechanics Steven L. Mielke, Ted Belytschko, and George C. Schatz p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p185 Modeling Self-Assembly and Phase Behavior in Complex Mixtures Anna C. Balazs p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p211 Theory of Structural Glasses and Supercooled Liquids Vassiliy Lubchenko and Peter G. Wolynes p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p235 Localized Surface Plasmon Resonance Spectroscopy and Sensing Katherine A. Willets and Richard P. Van Duyne p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p267 Copper and the Prion Protein: Methods, Structures, Function, and Disease Glenn L. Millhauser p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p299

ix

AR308-FM

ARI

23 February 2007

13:44

Aging of Organic Aerosol: Bridging the Gap Between Laboratory and Field Studies Yinon Rudich, Neil M. Donahue, and Thomas F. Mentel p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p321 Molecular Motion at Soft and Hard Interfaces: From Phospholipid Bilayers to Polymers and Lubricants Sung Chul Bae and Steve Granick p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p353 Molecular Architectonic on Metal Surfaces Johannes V. Barth p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p375 Annu. Rev. Phys. Chem. 2007.58:535-564. Downloaded from arjournals.annualreviews.org by ARIZONA STATE UNIV. on 01/10/08. For personal use only.

Highly Fluorescent Noble-Metal Quantum Dots Jie Zheng, Philip R. Nicovich, and Robert M. Dickson p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p409 State-to-State Dynamics of Elementary Bimolecular Reactions Xueming Yang p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p433 Femtosecond Stimulated Raman Spectroscopy Philipp Kukura, David W. McCamant, and Richard A. Mathies p p p p p p p p p p p p p p p p p p p p p461 Single-Molecule Probing of Adsorption and Diffusion on Silica Surfaces Mary J. Wirth and Michael A. Legg p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p489 Intermolecular Interactions in Biomolecular Systems Examined by Mass Spectrometry Thomas Wyttenbach and Michael T. Bowers p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p511 Measurement of Single-Molecule Conductance Fang Chen, Joshua Hihath, Zhifeng Huang, Xiulan Li, and N.J. Tao p p p p p p p p p p p p p p p535 Structure and Dynamics of Conjugated Polymers in Liquid Crystalline Solvents P.F. Barbara, W.-S. Chang, S. Link, G.D. Scholes, and Arun Yethiraj p p p p p p p p p p p p p p p565 Gas-Phase Spectroscopy of Biomolecular Building Blocks Mattanjah S. de Vries and Pavel Hobza p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p585 Isomerization Through Conical Intersections Benjamin G. Levine and Todd J. Martínez p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p613 Spectral and Dynamical Properties of Multiexcitons in Semiconductor Nanocrystals Victor I. Klimov p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p635 Molecular Motors: A Theorist’s Perspective Anatoly B. Kolomeisky and Michael E. Fisher p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p675 Bending Mechanics and Molecular Organization in Biological Membranes Jay T. Groves p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p p697 Exciton Photophysics of Carbon Nanotubes Mildred S. Dresselhaus, Gene Dresselhaus, Riichiro Saito, and Ado Jorio p p p p p p p p p p p p719

x

Contents