Atoms, Molecules, Electrons, Light and Heat in Nanometre Confinement

1.1 Introduction

The chemistry within yoctolitre (10-24L) holes or on the surface of particles with a yoctolitre volume is generally called "nanochemistry" because each dimension of these holes and particles measures a nanometre (nm): 1 yL = 1 nm. In volumes of up to 1000 yL, e.g. 10 × 10 × 10 nm, all distances between atoms and molecules on the particles or in the holes are within a few nanometres, which allows them to interact with each other and with ions and molecules that approach the surface. Holes left by missing single atoms, molecules or ions in yoctowell walls, and in the centres or surfaces of nano-particles or nanocrystals, can easily migrate through the whole species.

Yoctolitre dimensions are practical for the estimation of constituent numbers and models based on them: 1 yL can contain ~100 metal atoms, 50 molecules of metal oxide (MO), 33 molecules of water or 25 molecules of metal dioxide (MO2). Nanometre concepts are more appropriate if [111] surfaces have to be compared with [100] surfaces and where vectors, angles, and tenths of an Ångström become important.

The first special property of nanochemistry in or on yoctolitre-sized wells, pores, spheres or crystals depends on the fact that a large percentage of the atoms that form the holes or particles are surface atoms. Quite often the electrons on the hole or nanoparticle surfaces do not bind anything, but represent 'dangling bonds' with special activities. The second dominating fact is the relevance of space-dependent physical properties of molecules, which play hardly any role in bulk inorganic and organic chemistry. For example, glucose becomes water-insoluble in hydrophobic yoctowells, because the molecules have a hydrophobic edge, which sticks to the wall, and metallic Ag(0) nanocrystals seem to forget about their metallic electron cloud and fluoresce like covalent molecules, for reasons yet unknown.

The translation of stereochemistry into spin interactions of electrons and different kinds of magnetism, the chemical stabilization of electron-hole pairs in order to create luminescence of all colours by the variation of electric potentials, the change in metal crystal lattices on the surface of nanocrystals that renders them catalytically active or fluorescent, and the fixation of water-soluble carbohydrate edge amphiphiles in hydrophobic, water-filled yoctowells are subjects of current research, and chemists interested in the development of new properties should become familiar with them. Research in this field requires access to modern techniques such as rapid crystallization of vapour, laser ablation under water, preparation of atomically smooth surfaces, atomic force microscopy, electron transmission microscopy and electron scanning microscopy, among others.

Subjects covered in this book are, in order of increasing complexity:

• self-cleaning surfaces of nanometre roughness

• quantum dots (QDs), which provide everlasting colours and discrete energy levels instead of bands

• soft and hard magnetic particles for computers and other engineering uses

• minimization of functional AFM tips

• localization of many different single molecules in a very small aqueous space

• decomposition of chlororganic compounds in soils

• coupling of NMR signals with magnetic field steering

• fixation of proteins, DNA and cell surfaces on nanoparticles in water

• routine interconnection of nanoscale elements by nanowiring

• photolysis of water by sunlight

• synthesis of effective catalytic corners with different elements, particularly iron

• assembly of nanoparticles to form efficient thermoelectric elements

• minimization of computer hard disks.

Today's nanochemistry offers a unique chance to have a fresh look at the periodic system of the elements. Carbon chemistry, to name a popular example, is a huge field of classical and modern chemistry, but molecular coatings of nanoparticles that shrink in water on heating, graphite tubes showing ballistic electron transport or the dominance of dangling bonds in nanodiamonds, which makes them extremely reactive, are accessible only through nanochemistry and are of interest in industry and medicine. Magnetic data storage in computer or control devices, the conversion of sunlight into electric currents and of electric currents into light and refrigeration – all these processes depend on metallic nanoparticles and help to create or replace energy sources. Working with nanocrystals, nanowires and the inner walls of nanometre-wide wells and tubules in university labs will help young scientists to find rewarding problems for work in industry, which has to engineer, optimize, produce, sell and guarantee the nanosized constituents of computers, catalysts, light bulbs, solar converters, etc.

Most of the reference citations in this book date from 2005–2009 and are from easily accessible chemical journals. They provide a starting point for learning and research. Each important element of the periodic table has its own section, where relevant general properties are summarized and the element-specific ideas behind recent publications, together with selected results, are discussed. Altogether, the book is as "hands-on" as possible, and we have tried to provide the necessary physical and material chemical background in a descriptive manner. Nanochemists should know whether the products they develop in reaction flasks or on solid surfaces contain magnetic, luminescent or conductive layers, particles or wires.

Our aim is to make readers aware of current developments, while avoiding speculations about economic perspectives. The motivation for 'going nano' should always be scientific success, the hope of finding something new and surprising.

1.2 Water, Toluene, Nanoparticles and Nanocrystals

The most useful nanoparticles have diameters between 1 and 10 nm, sometimes up to 20 nm, i.e. volumes between one and a few thousands of yoctolitres. The 1 nm species are best for structural and theoretical studies, but 10 nm guarantees relatively high stability and longevity; 5nm is the standard compromise. Furthermore, all nanosized clusters of atoms or molecules that are of interest fulfil a function. The "functionality" may be as simple as being magnetic, or separation of sodium chloride into Na+ and Cl- ions, or as complex as the recognition and removal of two proteins from the bloodstream of a living organism.

We start with water, the lightest natural molecule on Earth with its molecular weight of 18. Its hexameric cluster is the lightest and most dynamic sub-nanoparticle, with a molecular weight of 6 × 18 = 108 Da. All molecules that are lighter than water, in particular hydrogen (H2) and helium (He), eventually disperse into outer space after reaching the...