/pdf/self-assembled-monolayers-of-thiolates-on-metals-as-a-form-1neb0hj3k4.pdf

Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

Thiolate-protected gold surfaces and interfaces, relevant for self-assembled monolayers of organic molecules on gold, for passivated gold nanoclusters and for molecule-gold junctions, are archetypal systems in various fields of current nanoscience research, materials science, inorganic chemistry and surface science. Understanding this interface at the nanometre scale is essential for a wide range of potential applications for site-specific bioconjugate labelling and sensing, drug delivery and medical therapy, functionalization of gold surfaces for sensing, molecular recognition and molecular electronics, and gold nanoparticle catalysis. During the past five years, considerable experimental and theoretical advances have furthered our understanding of the molecular structure of the gold-sulfur interface in these systems. This Review discusses the recent progress from the viewpoint of theory and computations, with connections to relevant experiments.

The gold–sulfur interface at the nanoscale

Microneedles for drug and vaccine delivery.

Building an electronic device using individual molecules is one of the ultimate goals in nanotechnology. To achieve this it will be necessary to measure, control and understand electron transport through molecules attached to electrodes. Substantial progress has been made over the past decade and we present here an overview of some of the recent advances. Topics covered include molecular wires, two-terminal switches and diodes, three-terminal transistor-like devices and hybrid devices that use various different signals (light, magnetic fields, and chemical and mechanical signals) to control electron transport in molecules. We also discuss further issues, including molecule-electrode contacts, local heating- and current-induced instabilities, stochastic fluctuations and the development of characterization tools.

Electron transport in molecular junctions.

Self-assembled monolayers (SAMs) of alkanethiols and dialkanethiols on gold are key elements for building many systems and devices with applications in the wide field of nanotechnology. Despite the progress made in the knowledge of these fascinating two-dimensional molecular systems, there are still several “hot topics” that deserve special attention in order to understand and to control their physical and chemistry properties at the molecular level. This critical review focuses on some of these topics, including the nature of the molecule–gold interface, whose chemistry and structure remain elusive, the self-assembly process on planar and irregular surfaces, and on nanometre-sized objects, and the chemical reactivity and thermal stability of these systems in ambient and aqueous solutions, an issue which seriously limits their technological applications (375 references).

Self-assembled monolayers of thiols and dithiols on gold: new challenges for a well-known system

The collisions of single platinum nanoparticles at an ultramicroelectrode were observed electrochemically by their characteristic current−time transients for a particle-catalyzed reaction. A single event is characterized by the current generated by an electrocatalyzed reaction of an indicator species (proton, hydrogen peroxide) present in solution. Since the indicator reaction does not occur at the selected ultramicroelectrode and can involve a high concentration of indicator species with a much larger diffusion coefficient than the nanoparticle, large amplification (10 orders of magnitude or more) in the current occurs. Every collision produces a unique current−time profile that can be correlated with the particle size, the particle residence time, and the nature of the particle interaction with the electrode surface. Applications to studying heterogeneous kinetics at single nanoparticles, determining particle size distributions, and as a very sensitive electroanalytical technique are suggested.

/pdf/observing-single-nanoparticle-collisions-at-an-3adj20c2jc.pdf

Observing single nanoparticle collisions at an ultramicroelectrode by electrocatalytic amplification.

We have studied electron transport properties of benzenedithiol and benzenedimethanethiol covalently bonded to gold electrodes by repeatedly creating a large number of molecular junctions. For each molecule, conductance histogram shows peaks at integer multiples of a fundamental conductance value, which is used to identify the conductance of a single molecule. The conductance values of a benzenedithiol and benzenedimethanethiol are 0.011 G0 and 0.0006 G0 (G0 = 2e2/h), respectively. The conductance peaks are broad, which reflects variations in the microscopic details of different molecular junctions. We have also studied electrochemical gate effect.

Measurement of Single Molecule Conductance: Benzenedithiol and Benzenedimethanethiol

Electrochemical hydrazine oxidation and proton reduction occur at a significantly higher rate at Pt than at Au or C electrodes. Thus, the collision and adhesion of a Pt particle on a less active Au or C electrode leads to a large current amplification by electrocatalysis at single nanoparticles (NPs). At low particle concentrations, the collision of Pt NPs was characterized by current transients composed of individual current profiles that rapidly attained a steady state, signaling single NP collisions. The characteristic steady-state current was used to estimate the particle size. The fluctuation in collision frequency with time indicates that the collision of NPs at the detector electrodes occurs in a statistically random manner, with the average frequency a function of particle concentration and diffusion coefficient. A longer term current decay in single current transients, as opposed to the expected steady-state behavior, was more pronounced for proton reduction than for hydrazine oxidation, revealing microscopic details of the nature of the particle interaction with the detector electrode and the kinetics of electrocatalysis at single NPs. The study of single NP collisions allows one to screen particle size distributions and estimate NP concentrations and diffusion coefficients.

/pdf/current-transients-in-single-nanoparticle-collision-events-51absi0xnp.pdf

Current transients in single nanoparticle collision events.

The electromechanical properties of a single molecule covalently attached to two gold electrodes are studied by simultaneously measuring the conductance and the force during the stretching of the molecule. The conductance, the spring constant of the molecular junction, and the dependence of the conductance on the stretching force are determined. Like the conductance, the spring constant of a molecule depends also on the molecule-electrode contacts. The forces required to break the molecule-gold contacts are 1.5 nN for alkanedithiols and 0.8 nN for 4,4' bipyridine, indicating that the breakdowns take place at the Au-Au bond and at the N-Au bond, respectively.

Measurements of Single-Molecule Electromechanical Properties

We have demonstrated a single molecule field effect transistor (FET) which consists of a redox molecule (perylene tetracarboxylic diimide) covalently bonded to a source and drain electrode and an electrochemical gate. By adjusting the gate voltage, the energy levels of empty molecular states are shifted to the Fermi level of the source and drain electrodes. This results in a nearly 3 orders of magnitude increase in the source-drain current, in the fashion of an n-type FET. The large current increase is attributed to an electron transport mediated by the lowest empty molecular energy level when it lines up with the Fermi level.

Xiaoyin Xiao

Papers

Observing single nanoparticle collisions at an ultramicroelectrode by electrocatalytic amplification.

Measurement of Single Molecule Conductance: Benzenedithiol and Benzenedimethanethiol

Current transients in single nanoparticle collision events.

Measurements of Single-Molecule Electromechanical Properties

Large gate modulation in the current of a room temperature single molecule transistor.