The miniaturization of components used in the construction of working devices is being pursued currently by the large-downward (top-down) fabrication. This approach, however, which obliges solid-state physicists and electronic engineers to manipulate progressively smaller and smaller pieces of matter, has its intrinsic limitations. An alternative approach is a small-upward (bottom-up) one, starting from the smallest compositions of matter that have distinct shapes and unique properties-namely molecules. In the context of this particular challenge, chemists have been extending the concept of a macroscopic machine to the molecular level. A molecular-level machine can be defined as an assembly of a distinct number of molecular components that are designed to perform machinelike movements (output) as a result of an appropriate external stimulation (input). In common with their macroscopic counterparts, a molecular machine is characterized by 1) the kind of energy input supplied to make it work, 2) the nature of the movements of its component parts, 3) the way in which its operation can be monitored and controlled, 4) the ability to make it repeat its operation in a cyclic fashion, 5) the timescale needed to complete a full cycle of movements, and 6) the purpose of its operation. Undoubtedly, the best energy inputs to make molecular machines work are photons or electrons. Indeed, with appropriately chosen photochemically and electrochemically driven reactions, it is possible to design and synthesize molecular machines that do work. Moreover, the dramatic increase in our fundamental understanding of self-assembly and self-organizational processes in chemical synthesis has aided and abetted the construction of artificial molecular machines through the development of new methods of noncovalent synthesis and the emergence of supramolecular assistance to covalent synthesis as a uniquely powerful synthetic tool. The aim of this review is to present a unified view of the field of molecular machines by focusing on past achievements, present limitations, and future perspectives. After analyzing a few important examples of natural molecular machines, the most significant developments in the field of artificial molecular machines are highlighted. The systems reviewed include 1) chemical rotors, 2) photochemically and electrochemically induced molecular (conformational) rearrangements, and 3) chemically, photochemically, and electrochemically controllable (co-conformational) motions in interlocked molecules (catenanes and rotaxanes), as well as in coordination and supramolecular complexes, including pseudorotaxanes. Artificial molecular machines based on biomolecules and interfacing artificial molecular machines with surfaces and solid supports are amongst some of the cutting-edge topics featured in this review. The extension of the concept of a machine to the molecular level is of interest not only for the sake of basic research, but also for the growth of nanoscience and the subsequent development of nanotechnology.

Artificial Molecular Machines.

Biomedical surface science: Foundations to frontiers

Microscopic view of epitaxial metal growth: nucleation and aggregation

A simple method is used to prepare highly monodispersed silver nanoparticles in the liquid phase, which starts from an initial synthesis in functionalized AOT reverse micelles. To narrow the particle size distribution from 43% to 12.5% in dispersion, the particles are extracted from the micellar solution. The size-selected precipitation method is used. The decrease in polydispersity of the silver nanoparticles is followed by transmission electron microscopy, by UV−vis spectroscopy, and by small-angle X-ray scattering. The nanocrystallites dispersed in hexane are deposited on a support. A monolayer made of nanoparticles with spontaneous hexagonal organization is observed. The immersion of the support on the solution yields to the formation of organized multilayers arranged as microcrystal in a face-centered cubic structure.

Synthesis of Highly Monodisperse Silver Nanoparticles from AOT Reverse Micelles: A Way to 2D and 3D Self-Organization

We review in this article the current theoretical understanding of collective and single particle diffusion on surfaces and how it relates to the existing experimental data. We begin with a brief survey of the experimental techniques that have been employed for the measurement of the surface diffusion coefficients. This is followed by a section on the basic concepts involved in this field. In particular, we wish to clarify the relation between jump or exchange motion on microscopic length scales, and the diffusion coefficients which can be defined properly only in the long length and time scales. The central role in this is played by the memory effects. We also discuss the concept of diffusion under nonequilibrium conditions. In the third section, a variety of different theoretical approaches that have been employed in studying surface diffusion such as first principles calculations, transition state theory, the Langevin equation, Monte Carlo and molecular dynamics simulations, and path integral formalism...

Collective and single particle diffusion on surfaces

We describe the construction and performance of a gas condensation cluster source. The source was designed for deposition of mass-selected metal clusters with controlled landing energy. We have produced clusters of Pbn (n=2–∼300) and Agn (n=∼20–∼300) with sufficient intensity to deposit size-selected clusters to a density of 1012 clusters/cm2 in 10 min. The landing energy of the clusters can be controlled from ∼25 to 800 eV.

Gas condensation source for production and deposition of size-selected metal clusters

We have investigated the early stages of film growth via deposition of size‐selected silver clusters on graphite, as a function of incident cluster size. For all sizes from 50 to 250 atoms per cluster, the deposited clusters are mobile and coalesce into three‐dimensional particles with a ‘‘universal’’ diameter of ≊14 nm, possibly a consequence of lattice strain between the silver and graphite. The 14 nm particles are found mainly in small aggregates, indicating that they themselves are to some degree mobile. We have also found evidence that the particle mobility is influenced by the cluster impact angle.

Diffusion and aggregation of size-selected silver clusters on a graphite surface

We have investigated the growth of three‐dimensional Ag particles at atomic steps on the surface of highly oriented pyrolytic graphite using a scanning electron microscope. By controlling the growth parameters the cluster growth was confined to the steps avoiding terrace nucleation. In this way quasi‐one‐dimensional chains of Ag nanoclusters of approximately 10 nm diam were produced. The results suggest the viability of an important new route to the creation of controlled nanoscale structures. A comprehensive surface study indicates that cluster mobility and coalescence play an important role in the growth mechanism on the steps. Evidence was also found that the graphite surface has several different types of surface steps. A quantitative analysis of the cluster distribution on the steps was performed, to investigate the nucleation and growth processes at temperatures from 50 to 205 °C.

Diffusion controlled growth of metallic nanoclusters at selected surface sites

The growth of silver and gold clusters following atomic vapour deposition on highly oriented pyrolytic graphite has been studied using scanning electron microscopy and scanning tunnelling microscopy (STM) Three-dimensional clusters were grown on the terraces and quasi-one-dimensional chains of clusters along the surface steps An STM study was made on the effect of the step height on cluster nucleation Charge-density modulations on the substrate surface around the silver clusters were analysed A preliminary study of the deposition of mass-selected silver clusters from a beam onto the graphite surface has been made The effect of the impact energy of silver clusters on the deposition process is explored

Deposition and growth of noble metal clusters on graphite

We have used the scanning tunnelling microscope (STM) to investigate the deposition of size-selected (Agn)+ clusters, in the size range 50-400 atoms, on highly oriented pyrolytic graphite (HOPG). The cluster morphology has been explored as a function of impact energy. For high deposition energies (~10 eV/atom) the clusters are pinned to the surface and flattened on impact. For lower impact energies (~1 eV/atom) the footprint of the cluster on the surface is significantly reduced. By controlling the deposition parameters clusters have also been selectively sited at surface steps. Larger particles, arising from diffusion and aggegation of deposited clusters, are visible with the scanning electron microscope (SEM) but not the STM; we find definite evidence that the interaction with the STM tip disturbs these particles.

http://iopscience.iop.org/article/10.1088/0953-8984/8/41/025/pdf

L. Kuipers

Papers

Gas condensation source for production and deposition of size-selected metal clusters

Diffusion and aggregation of size-selected silver clusters on a graphite surface

Diffusion controlled growth of metallic nanoclusters at selected surface sites

Deposition and growth of noble metal clusters on graphite

The impact of size-selected Ag clusters on graphite: an STM study