Chemistry in Nanotechnology: Particles, Bonds and Structure

Let us make an in-depth study of the chemistry in nanotechnology. The below given article will help you to learn about the following things:- 1. Particles and Different Types of Chemical Bonds in Nanotechnology and 2. Chemical Structure of Nanotechnology.

Particles and Bonds:

Chemical Bonds in Nanotechnology:

In addition to the elementary composition, the interactions between the atoms determine the properties of the materials, and therefore, of devices. So knowledge of chemical bonds in a device is essential for its functioning. In classical technology, bond properties are de­scribed as collective phenomenon, and gen­eral material parameters are utilized for a fairly indirect characterization. Apart from the properties known from the bulk materials, surface and interface-prop­erties in particular exert an increasing influ­ence as structure size decreases.

The material properties in microscopic dimensions often differ dramatically from bulk properties. Be­sides the dominant role of surfaces and inter­faces, the individual bond is also no longer negligible relative to the sum of the bonds in the structure. Often individual bonds or sin­gle molecules determine the properties and function of a nanostructure.

In general, all types of positive interactions between particles represent bonds. Interac­tions between atoms, groups of atoms, ions and molecules can vary widely with respect to their character and their strength. To dif­ferentiate, these interactions were divided into classes known as bond types. These classes are well suited for a description of bonds. In contrast to classical synthetic chemistry where strong bonds are important, often the medium and weak bonds are of particular im­portance in nanotechnology.

The importance of weak bonds increases with the increasing size of the aggregates constructed, which is comparable to what happens in nature. While in the field of strong bonds the differentia­tion of bond types is easy, the area of weak bonds is determined by the parallel existence of several interactions with a wide range of strengths and characters.

Molecular geom­etries are not just described by the topology of covalent bonds. Other types of bonds as well as weak interactions contribute substan­tially to the establishment and conservation of given geometries, and, therefore, have to be considered. Thus, the following sections will introduce the key classes of chemical bonds and discuss their importance to nanotechnol­ogy.

Van der Waals Interactions:

All of the shells of atoms interact with each other. When atoms approach each other, the electrons of one atom deform the distribu­tion of the electrons of the other atom. This deformation disturbs the charge distribution in a way such that the sum of the energy of the two approaching atoms is lower than the sum of the atoms initially. This difference in energy determines the strength of the bond.

If this effect is not influenced by other bonds (e.g., by the exchange of electrons), the bond energy is fairly low. The Van der Waals bond is a weak bond. At room temperature, the bond between individual atoms can be easily thermally activated and broken.

Van der Waals is, nevertheless, of par­ticular importance in nanotechnology, be­cause the building units are usually solids and consist of molecules instead of individual atoms. If two or more atoms connected by strong ionic, covalent, coordinative or metal bonds, then the interactions of the electron shells with surfaces and molecules are in conjunction with the Van der Waals bonds.

As a result, as the number of atoms in a mol­ecule increase, this molecule is able to bind to a substrate based solely on Van der Waals bonds. One consequence of this effect is the decreasing vapor pressure of homologue compounds with increasing molecular size.

The van der Waals bond is, therefore, a ba­sic type of bond, which becomes important due to the cooperative effect of many atoms bound to each other. The mobility of mole­cules is determined by the size. Another pa­rameter is the partial dissociation of Van der Waals bonds by intermolecular movements.

If atoms or groups of atoms are only connect­ed by freely rotating bonds, the rotation of one part of the molecule can thus induce the separation of the respective bond. With fixed bonds, all bonds are distributed in a coop­erative manner.

Van der Waals bonds play an important role in hydrophobic interactions. They are essential in resist technology and therefore in the whole field of micro lithog­raphy. They are also essential for living cells, especially in the creation of the three-dimen­sional structure of proteins.

In cells, hydro­phobic interactions are a prerequisite for the composition of lipid bilayer membranes and the inclusion of membranous proteins in these layers. In analogy to such structures in nature, van der Waals interactions are impor­tant in nanotechnology, especially in the field of supra-molecular chemistry for the arrange­ment of complex molecular aggregates based on smaller units.

Dipole-Dipole Interactions:

Owing to the differences in electronegativ­ity, molecules consisting of different atoms normally exhibit an inhomogeneous electron distribution. Only when the bonds are sym­metrical is this distribution not apparent in the surroundings. Otherwise, an electrical polarity of the molecule is observed. Such molecules, with one or more dipole moments, attract each other. The intensity of polarity determines the strength of the dipole-dipole interaction.

Dipole-dipole interactions are also ob­served in cases when only one half exhibits a permanent dipole moment. Because the electron shell can be deformed by external fields, a molecule with a permanent dipole moment is able to induce a deformation and therefore a polarization resulting in a dipole moment (Fig. 1).

The energy gain in such cases is usually lower than the interaction of permanent dipole moment. The bond energies are determined by the tendency of the electron shell to be polarized. If this ca­pability is low (hard shells), only weak dipole moments are induced, the resulting bond is, therefore, weak. In shells with high capabil­ity for polarization (soft shells), significant dipole moments can be induced.

Dipole-dipol interactions are widely dis­tributed. They account for undesired effects in micro technology, because they are respon­sible for unspecific interactions. These inter­actions result in deposition on surfaces or unspecific binding of individual molecules/particles (Fig. 2). In particular, electron-rich heavy atoms exhibit readily polarizable elec­tron shells, so that they are sensitive to un­specific adsorption. In gas reactors, such as vacuum equipment, unspecific adsorption is minimized by the heating of reactor surfaces and substrates, through thermal activation of desorption.

Tighter bound particles on sub­strates are treated by etching through sput­tering, which is not applicable for substrates, such as in the case of substrates with ultrathin and molecular layers. To counteract these processes of undesired adsorption in liquid phase processes, ultra-pure substances and solvents are used.

Coupled dipole bonds are utilized in the three-dimensional folding or arrangement of synthetic macro- or super molecules. The ap­plication of less specific bonds for the design of molecular Nano architecture in nanotech­nology is still a long way off the level demon­strated in nature.

Ionic Interactions:

Where there are large differences in the electro-negativities of atoms, a transfer of one or more electrons from the less to the more elec­tronegative interacting partner is observed. The resulting bond is not determined by the bonding electrons, but by the interactions of the ions created by the electron transfer. The strength of this bond is comparable to a covalent bond; it is, therefore, a strong chemical interaction.

Pure electrostatic interactions between ionized atoms, as in the case of salts, are of less interest in nanotechnology. In contrast, molecular ions and also polyions are of par­ticular interest. Macromolecules often exhibit a multitude of similar functional groups.

If these groups are ionizable and can be readily ionized (e.g., as a result of dissociation pro­cesses), this effect results in polyionic macro­molecules. They can interact with small ions of opposite charge, but also with similarly charged polyionic partners, resulting in the creation and stabilization of multiple ultra-thin layers or complex molecular aggregates.

Surface charges, electrostatic repulsion and electrostatic bonds are essential for the ma­nipulation of macromolecules, super-molecular aggregates, micelles and nanoparticles in the liquid phase. Nano heterogeneous systems can be created, stabilized or collapsed by ad­justment or compensation of surface charg­es.

Metal Bonds:

The creation of strong chemical bonds by exchange of binding electrons can also take place without asymmetric distribution of the electron density. If the exchange occurs only in one direction, a single covalent bond is created. If the exchange takes place in several spatial directions and is furthermore com­bined with a high mobility of the binding electrons, a so called metal bond is created.

Through the simultaneous existence of bonds in various spatial directions the metal bond is present in a three-dimensional net­work of equal bonds. Clusters are created where a limited number of atoms are involved. For large numbers of atoms, an extended binding network leads to a three-dimensional solid.

Owing to the high mobility of the bind­ing electrons, this solid is electrically conduc­tive (Fig. 3).

The metal bond is of special interest in mi­cro and Nano technology due to the broad application of metals and semiconductors as electrical or electronic materials. Addition­ally, metal bonds facilitate the adhesion and both electrical and thermal conductivity at interfaces between different metals and in­side alloys.

Completeness or discontinuity of metal bonds in the range of molecular di­mensions inside ultrathin systems determine the Nano technological functions, such as tunnelling barriers realized by local limitations of the electron mobility or the arrangement of ultrathin magnetic layers for magneto-resistive sensors leading to a change in magnetic properties at constant electrical conductivity.

Covalent Bonds:

Strong bonds occur in the interaction of two atoms with unpaired electrons, resulting in doubly-occupied binding orbitals (Fig. 3). While the density distribution of electrons does not differ significantly from the density distribution of the free atoms, the differences in the electronegativity of the binding atoms results in polarity for covalent bonds.

In contrast to the typically extended solids in the case of the metal bond in some cases the co­valent can lead to particles consisting of only two atoms, e.g., oxygen or nitrogen found in the air. Covalent bonds can also affect just several or a high number of atoms. So the results can be linear, disk-shaped, globular molecules or solids extended in three dimensions.

The fixed rules for the electron density dis­tribution are of importance in nanotechnol­ogy, these rules being based on the number of possible bonds per atom, the number of non-binding outer electrons, and the angle between the bonds.

They apply for all bonds types with electron exchange as essential distribution to the bond, such as polar and apolar atomic bonds, coordinative bonds and hy­drogen bonds. These bonds are directed. The geometry of the bonds around an atom in influenced by its valence. Bivalent atoms create linear or bent structures and trivalent atoms result in trigonal-planar or trigonal-pyramidal geometries (Fig. 4).

Regular ge­ometries around atoms with four valences are planar, square or tetragonal, which are deformed in the case of asymmetric substitu­tions. Square pyramids are typical for five va­lences, and octahedral or trigonal pyramids for six valences (Fig. 5).

The three parameters ‘valence’, ‘polarity’, and ‘direction’ create a complex set of rules for the architectural arrangements based on covalent bonds. The orientation and arrange­ments of bonds determine not only the topol­ogy of bonds, but also the mobility of atoms and groups of atoms relative to each other.

So the sum of bonds affects how the bond topol­ogy determines a certain molecular geometry or allows degrees of freedom for spontaneous activated intermolecular mobility, and how the external pressure affects the mechanical relaxation of molecules. Also, without intermolecular bridges the free rotation of bonds could be limited due to double bonds.

The creation of molecules nanostructures relies on the degrees of freedom of individ­ual bonds on the one hand, and the rigidity (limitation of mobility) of certain parts of the molecules on the other hand. Hence double bonds, bridged structures and multiple ring systems of covalent units are important mo­tifs for the molecular architecture in molecu­lar nanotechnology.

Coordinative Bonds:

Bonds are created by the provision of an elec­tron pair by one of the binding partners (the ligand) for a binding interaction. A prereq­uisite is the existence of double unoccupied orbitals at the other binding partner, so that a doubly occupied binding orbital can be cre­ated. According to the acid-base concept of Lewis, electron donors are denoted as Lewis bases, electron acceptors as Lewis acids.

The central atom in such a coordinative bond is usually the respective acid; the ligands are the Lewis bases. Such coordinative bonds are typically found with metal atoms and metal ions, which always exhibit unoccupied orbitals. Thus metal atoms or ions in solution usually exist in coordinated interactions.

The metal central ion or atom (central particle) is surrounded by a sphere of several ligands and creates a so called complex; therefore these bonds are also denoted as complex bonds. The stability of complex bonds lies between the strength of the weaker dipole-dipole interactions and of covalent bonds, thereby covering a wide range.

Coordinative bonds are, therefore, particularly well suited to the realization of adjustable binding strengths and thus to adjustable lifetimes of molecules. This is of great importance for construction in supra-molecular architecture.

Nature also uses this principle of finely tunable binding strengths of complex bonds, e.g. in the Co-or Fe-complexes of the heme groups of enzymes. Similar to the covalent bonds, the coordina­tive bonds are also coupled to the spatial ori­entating of the binding orbitals (Figs. 4 and 5). Because the central particles are usually involved in two or more bonds, their orbitals determine the geometries of the complex compounds.

Two or multiple-valent ligands often bind on one and the same central par­ticle. When multiple binding ligands interact with several central particles, complexes with several cores are created. Such compounds are promising units for supra-molecular ar­chitectures and therefore of special interest in molecular nanotechnology.

Beside anions and small molecules, ring- shaped molecules, extended molecules and parts of macromolecules can also act as li­gands. Covalent and coordinative bonds are then both responsible for the resulting mo­lecular geometries. Because the central par­ticle and often also the ligands are ions, co­ordinative bound architectures in addition to exhibiting complex and covalent bonds also display ionic and dipole-dipole interactions, representing a complex structure.

Hydrogen Bridge Bonds:

The hydrogen bond is a specific case of a po­lar covalent interaction. It is based on hydro­gen atoms, which create interactions between two atoms of fairly strong electro-negative elements. In this way, one of the atoms is rela­tively strongly bound as a covalent binding partner, and the second significantly weak­er.

A classic case of hydrogen bonds occurs in water, where they are responsible for the disproportionately high transition points of water. Hydrogen bonds are observed when the bond of a different atom to hydrogen is so polar that the separation of the hydrogen atom almost certainly occurs. So oxygen and nitrogen, and to a certain degree sulfur also, are the preferred binding atoms for hydrogen bonds.

The individual hydrogen bond is of relative­ly low energy, distributing only a weak con­tribution to the overall energy. In addition, it is easily cleaved. However, several hydrogen bonds between two molecules can stabilize the created aggregate significantly by inducing a cooperative binding.

Hydrogen bonds lead to less specific ad­sorption processes; therefore they belong to the class of bonds responsible for distur­bances at surface modifications or on layer deposition. In contrast to Van der Waals bonds and dipole-dipole interactions, hydro­gen bonds are localized and oriented.

So that they contribute significantly to specific in­teractions. In this respect, they are similar to coordinative bonds. So hydrogen bonds play an important role in both the supra-molecular chemistry and the super-molecular synthesis of biomolecules.

Polyvalent Bonds:

Nanotechnology is based on the creation and dissociation of connections due to interac­tions between atoms or molecules. Reduced dimensions result in a lower relative precision for external tools, so the accuracy of manipu­lations has to be realized by the specificity of chemical bonds instead of by external means.

A fine-tuned reactivity is required, which is not possible with the limitations of the in­dividual bonds. Instead, through the differentiation of a few types of discrete individual bonds, chemical reactivity and specific stability can also be achieved with a digital binding prin­ciple, characterized by the arrangement and number of bonds determining specificity and stability.

The energy of the individual bond has to be sufficiently small, so that it does not result in a stable final binding and can be dis­sociated if needed. Van der Waals bonds fulfil the requirement of weak interaction energies, but they do not exhibit positional specificity.

So they are not ideal for digital binding, and participate only as background bonds. The requirements of both low binding energy and positional spec­ificity are met by many coordinated interac­tions as well as by the hydrogen bridge bond. These two bonds, therefore, play a central role in the realization of molecular and su­pra-molecular architecture in living systems. Additionally, the arrangement of a synthetic Nano architecture depends on these bonds.

In such systems, the strength of an individ­ual bond matters less than the number, posi­tion and relative mobility of binding groups, which determine the geometry and stability of larger molecular architectures. While individual weak bonds are easily broken, a cooperative effect occurs in the case of coupled bonds, when several bonds only dissociate together. This phenomenon is well- known from the melting behaviour of dou­ble-stranded DNA.

The thermally induced separation of the two strands connected by hydrogen bridge bonds requires increased temperature with increased strand length and a higher density of hydrogen bridges (GC/AT ratio). Over a length of about 40 bases, the melting temperature does not increase fur­ther, pointing to an independent movement of strand sections above a critical length.

The mobility of molecular groups deter­mines the size of cooperative effective sec­tions in larger molecule, which are able to bind externally in a polyvalent manner. The cooperative sections can be extended by the inclusion of rigid groups, such as conjugated double bonds, bridges based on dipole-dipole interactions, or coordinated interactions. This is a prerequisite for stabile polyvalent interactions between large molecules based on multiple weak bonds.

Natural molecular architectures demonstrate the synergetic use of different bond types. So the binding pock­ets of enzymes or antibodies often utilize a complex system of hydrogen bridge bonds, dipole-dipole interactions, and Coulomb and van der Waals interactions.

The strength of polyvalent bonds consisting of one type of individual bond is determined by the strength of the individual bond and the number of bonds connected by the rigidity of the molecule. So building units with a high rigidity and a compatibility with the liquid phase are of specific interest in Nano technology.

Linear aliphatic polymers do not fulfil these requirements without the introduction of groups for additional rigidity, in contrast to biological macromolecules, such as dou­ble-stranded DNA or protein. In synthetic chemistry, rigid and connected macro cycles are appropriate candidates.

Other interesting materials are substituted metal clusters, nanotubes and other nanoparticles. They pro­vide an extremely high rigidity based on the strong bonds between the atoms of the cluster or the particle, resulting in a coupling of sur­face bonds as regards mobility. In such cases, the interactions of groups of weak individual bonds represent polyvalent bonds.

Polyvalent bonds, which are strongly cou­pled weak bonds, provide a base for Nano architectures. While the activation barrier for the establishment and the dissociation of individual bonds is low, the simultaneous ac­tivation of a group of coupled weak bonds is extremely unlikely.

So, after creation, aggre­gates are stabile in the long term. Only under extreme conditions—or as a result of factors that assist in the successive opening of the weak individual bonds (e.g., the catalytic ef­fect of an enzyme)—can polyvalent bonds be reversed (Fig. 6).

The synthetic challenge for molecular Nano architecture is to avoid the creation of complex three-dimensional polymeric networks by spontaneous aggregation, but to control the aggregation so that, in every step, individual units are assembled at defines positions. A pre­requisite is a high efficiency in the coupling reactions combined with a low probability for competing reactions.

Chemical Structure:

Binding Topologies:

A relationship between the internal geometry of molecules and the coordinates the exter­nal reference system is essential for nanotech­nology. In the following, the basics of chemi­cal structure are discussed from the viewpoint of spatial determination. Often, terms such as ‘structure’ and ‘molecule’ are not sufficient to describe all the geometric and dynamic as­pects of the interactions of molecules with Nano technological structured surfaces and the construction of supra-molecular architec­tures on the surfaces of solids.

The external and internal mobility of molecules as well as the full effect of strong and weak interactions have to be included. The term ‘molecule’ will be discussed in the gas phase. Here, all atoms with the same (averaged) directional components of translation form a molecule. The joint movement is based on interatomic binding forces. The proximity alone is not sufficient as a param­eter, because there are conformations in mol­ecules with rotating bonds where atoms are in a close proximity but are without a direct strong bond.

Also, the absolute strength of a bond is not sufficient for a description. There are molecules in the gas phase held together by hydrogen bonds, e.g. acetic acid, which mists in the gas phase as a dimer. The crite­rion of common translation vectors can be transferred to the liquid phase.

However, it is not applicable when the translation of the particles is hindered, e.g. in solidified matri­ces. For this reason, the relative (instead of the absolute) strength of binding topologies will be used for the characterization of a particle. A binding topology includes a linear arrange­ment or a network of bonds, which in its entirely is more stable than all other bonds through atoms in its proximity.

It is indepen­dent of the type of bonds, and also weaker in­teractions such as hydrogen bridge bonds or cohesive forces are included. This approach allows the general discussion of single mol­ecules, micelles and particles. An estimation of the strength of the bind­ing topologies requires the discussion of a particle and the environment as one system. Strong particles exhibit stronger individual internal bonds compared with weaker exter­nal ones.

In this sense, a small alkane (such as ethane in the condensed phase) represents a very strong unit. Transformation into the gas phase is easy, in contrast to the significantly higher temperatures required for breakage. The transformation of long-chain molecules of polyethylene, which have the same cova­lent bonds as in ethane, into a mobile phase requires strong thermal activation (melting) or the substitution of the solid-state interac­tions between the molecular chain by inter­actions between dissolved molecules and solvent molecules (solvation).

Mechanical forces lead to the breakage of the covalent bonds, but not to an extraction of a molecule as a unit from the solid. The sum of the weak interactions with the environment is stronger than an individual intermolecular bond in the topology.

The movable macromolecule is a relatively weak unit in the binding topology. Molecules with a large of internal stabilizing interactions represent a stronger unit that the unfolded molecule. It is not the strength of a covalent bond network alone, but the sum and the arrangement of all intermolecular in­teractions that determine the binding topol­ogy of a particle. Nanotechnology utilizes different levels of internal stabilization of particles to realize durable devices with strong bond structures.

The different technological steps use units with a wide range of strengths. The stabil­ity criterion is the lifetime. For a successful device all components must be functionally preserved over the whole lifetime, which is typically in the range of years. When creating Nano architectures, intermediate units have to be stabile only for the given process step, which can be in the second or even millisec­ond range. Single units used in the technolo­gy frequently possess the character of reactive intermediates with even shorter lifetimes.

Particles with shorter lifetimes include mo­lecular aggregates with weaker bonds, such as van der Waals or hydrogen bridge bonds, as the interactions connecting the subunits. Typical examples are micro emulsions and micelles. Also, coordinated compounds are relatively unstable aggregates as in the case of the high exchange rates of ligands.

The geometry of rigid molecules is de­termined completely by the binding topol­ogy. This applies to solids with dense three-dimensional binding networks, but also to molecules consisting of two atoms, small linear molecules with multiple atoms such as carbon dioxide, simply bent molecules such as water, and highly symmetrical molecules such as benzene.

Various conformations of one and the same molecule represent different geometries at the same binding topology. With an increase in the number of free rotating bonds, the num­ber of possible geometries of particles with same binding topology also increases.

The internal mobility of particles represents a challenge to Nano construction. Chemi­cal stability does not imply spatial stability. Mobility required in coupling steps could be incompatible with certain functions or with subsequent steps in the synthesis.

The restric­tion of degrees of freedom of mobility is an essential instrument for molecular nanotechnology. On the other hand, internal mobility of particles is also an important instrument, because many chemical and physical func­tions require mechanical flexibility. Func­tional Nano architectures call for balanced and not maximal mobility.

Building Blocks of Covalent Architecture:

Ideal approaches to molecular nanostructures that have covalent bonds utilize pre-synthesized units (which are easily prepared and manipulated in a homogeneous mobile phase) and their coupling to substrate surfaces.

This general principle correspond to the classical mechanical construction approach, which builds complex units from prefabricated building blocks, or to traditional solid phase synthesis, which builds chain molecules by subsequent coupling of molecular groups. Molecules consisting of covalent bonds can be grouped according to formal constructive properties.

Even complex binding topologies have their roots in a few basic types of units (Figs. 7 and 8):

i. Single binding elements (‘terminators’), e.g. alkyl or trimethylsilyl groups

ii. Double binding elements (‘chain ele­ments’), e.g., alkenes, simple amino acids

iii. Three-fold binding elements (‘branches’), e.g., substituted amino acids

iv. Four-fold and higher branched elements

There are a large number of multiple branched elements. They can usually be traced back to a combination of units from the above mentioned first three classes. So the three-fold

binding phlotoglucin (1, 3, 5 trithoxy ben­zene) can be thought of as being assembled from three chain elements (CH) and three branches (COH).

There are three types of chain elements, based on the symmetry of the coupling group (Fig. 9, A-C):

i. Two identical coupling groups, e.g., alkane diol (A)

ii. Two different groups complementary to each other, e.g., amino acids (B)

iii. Two different and not complementary groups, e.g., amino alcohols (C)

The branches can be divided into six basic types (Fig. 9, D-I):

i. With three identical coupling groups, e.g. glycerol (D)

ii. With two identical and a third, comple­mentary, group, e.g. lysine (E)

iii. With two identical and a third, non-complementary, group, e.g. diamine alcohol (F)

iv. With three different groups, including two complementary to each other, e.g. tyrosine (G)

v. With three different groups, with one com­plementary to the two other, e.g. hydroxyl al-kyl amino acids (H)

vi. With three different non-complementary groups (I)

Silicon and carbon are well-suited as units for the construction of complex three-dimensional architectures due to their four valenc­es. In contrast to carbon, with its stabile C—C bond and a wide variety of chemical methods for preparation and manipulation, in silicon structure Si—C and Si—O bonds prevail. The carbon atom is the center of an elementary tetragon in the sp^{3 _}hybridized state; the same basic geometry is formed by Si(O), tetrahe­drons. Both structures are responsible for highly branched spatial structures. In its sp²– hybridized state, carbon represents a simple branch leading to planar structure.

The properties of molecular systems com­bine the fairly design-oriented aspects of the planned architecture on one side with the rather technological aspects on the other. The binding topology is determined by the num­ber of coupling groups per unit, which is in­fluenced by the potential of internal connec­tions. In contrast, the symmetrical properties of the units determine the choice and the or­der of reactions leading to the architectural arrangements.

Units for a Coordinated Architecture:

The scheme for covalent bonds is also ap­plicable to other types of bonds. The central atom of coordinated compounds typically exhibits ligand numbers of between 4 and 6. When the ligands with additional coupling groups are bound in a stabile manner to the central atom, complex compounds can act as a chain element or branch.

Complexes consisting of monovalent li­gands and with a complete saturation of the electron vacancies frequently exhibit only a low stability. Multiple-valent ligands stabilize to a significant extent through the distribu­tion of electron pairs from two or more donor groups. Such so-called chelate complexes are well suited as units for supra-molecular archi­tecture.

The geometries of molecular groups based on complex compounds are determined by the symmetries of the electron shells of the central atom, which are determined by the atomic number and the degree of ionization of the central atom.

In general, metals po­sitioned further left and low in the Periodic Table create coordination spheres of higher numbers than metals from the top right. An additional point affecting the geometries of complex architectures is that the overall number of coupling groups of a coordina­tive compound is related to the ratio of the number of coupling groups are ligand to the valence of the ligands inside the complex.

So a six-fold coordinated central atom and three bivalent ligands with one external coupling group each results in a three-fold coupling complex, which is a simple branch. Changes to the oxidation number of the central atom affect not only the stability of the individual coordinative bond, but often the geometry of the coordination shell also.

Chelate ligands with four or more electron pair donor groups are able to build stabile che­late bonds with several central atoms simulta­neously, thereby creating stabile multiple core complexes or polymeric complex structures.

Another route to multiple-core complexes is the subsequent reaction of ligands with each other (such as additions onto double bonds or condensation) while preserving the coordi­native bond. Helical supra-molecules known as ‘helicates’ can be constructed through the combination of multiple valent bridge ligands with multiple central atoms.

Building Blocks for Weakly Bound Aggregates:

In addition to covalent and coordinative bonds, dipole-dipole interactions, hydrogen bridge bonds and Van der Waals bridges can also lead to the assembly of molecular build­ing blocks for nanotechnology. Normally (ex­cept at very low temperature) a single bond is not sufficient to stabilize a particle consisting of several atoms. So it is preferable that poly­valent bonds are involved, usually exhibiting a mixture of the different bond types.

The collective effect of weak bonds is en­hanced when the participating particles themselves consist of several atoms bound together by strong bonds. Such a building block demonstrates a general principle of a binding hierarchy: with the increasing size of the molecular aggregate the average strength of the bonds between the particles decreases, and the number of simultaneous bonds rises (Fig. 10).

Many particles in the liquid phase, which influence the reaction but will not actually be involved in the final molecular structure that is created, are bound by weak and shorter- lived interactions. These particles include the solvent molecules, and also other small mol­ecules, which determine the rate and selectiv­ity of the chemical reactions by preferred in­teractions with certain regions of the reacting molecules and solid surfaces.

Typical exam­ples are small amounts of competing solvent or surface-active substances, and also metal ions that stabilize reactive intermediates by short-lived coordinative interactions or by influencing the concentration of free ligand groups. These small molecules, therefore, act by assisting the important molecules in the creation of molecular nanostructures.

Assembly of Complex Structures through the Internal Hierarchy of Binding Strengths:

The general principle of binding strength hierarchy underlying complex molecular structures has already been demonstrated in biology (Fig. 11).

Proteins show the strong modularity applied by nature in the Nano world: the basic structure of the complex molecule is one-dimensional (primary struc­ture) and this structure is created by covalent (strong) bonds. The secondary structure is two- or three- dimensional and is stabilized by multiple hydrogen bridges as well as dipole-dipole inter­actions, applying bonds that are individually weak but sufficient to give significant support to the structure.

The tertiary structure is al­ways three-dimensional and is preserved by a number of bonds. At the same time, mobility is induced through the low activation barri­ers of many weak bonds. The building blocks used for the primary structure are limited in number (20) and chemistry (amino acids).

Every block consists of four elemental units that have specific tasks, with three of these being identical in all blocks: a coupling unit (carboxyl group), a second and complemen­tary coupling group (amino group), a central connective unit (methane group of central carbon) and a variable unit (bound to the central carbon).

These four units are con­nected by stronger bonds (resistant to hydro­lysis) compared with the bonds between the individual building blocks (peptide bonds ion-resistant to hydrolysis). Using this hierarchy and enzymes as high­ly specific molecular tools, nature has suc­ceeded in the realization of highly complex, three-dimensional structures of thousands of atoms in a highly defined and function­ally optimized manner, without the need for macroscopic tools.

Only the modular ar­rangement in combination with control of the binding strength and the resulting intermolecular mobility (and through the application of self-assembly mechanisms) overcomes the optimization problem.

Classical and also supra-molecular synthetic chemistry are not yet able to apply this strat­egy of bio-molecular systems to other classes of compounds and new functions. They do, however, use an analogue of connecting modules of molecular constructions with de­creasing bond strengths through increasing unit size.

Reaction Probability and Reaction Equilibrium:

A chemical reaction occurs when the two re­action partners come close to each other and the sum of the relative movements overcomes the activation barrier. Thus the reaction is closely connected with both the internal and the external movement of the molecules.

In an ideal gas, the reaction probability for par­ticles of a particular geometry is based on the spontaneous distribution of energy on the degrees of freedom of mobility. In addition to the number of particles N in a given volume V and the frequency factor f (which includes the geometric properties), the ratio of the mobility energy (RT) to the activation barrier (E) determines the reaction rate r (Arrhenius equation).

For a reaction that includes only one partner, for example in decay processes, the following applies:

In classical chemistry, the reaction rate is a result of the sum of the reaction probabili­ties of the individual molecules. Because of the huge number of particles, even for small concentrations and volumes (e.g. 600 000 000 molecules in 1 pi in a 1 Nano-molar solution), individual reaction probabilities are negligi­ble relative to the overall reaction rate.

In nanotechnology, small assembles of par­ticles—sometimes even a single particle—do play an important role. A cube of 10 nm— and even a higher concentration of 10 mmol L^_1 —contain only six particles on average. The lifetime of a Nano technical device can be estimated from the ratio of the reaction probability to the tolerable failure rate.

If an individual molecule plays a key role and de­termines the functionality, the effects of the reaction probabilities are of particular impor­tance. Fast decays or other competing reac­tions should have a lower probability than the planned molecular reaction. This principle leads to a strict avoidance of competing reac­tions because of the absence of potential reac­tion partners for such reactions. This rule is softened for individual elements with a func­tion realized by several parallel processing molecules, thereby correlating with the num­ber of tolerable malfunctions per individual element.

However, nanotechnology has the means to overcome this problem. An important tool to manage reaction probabilities is the control of the spatial movement of particles. While in classical chemistry this movement is based on spontaneous thermal mobility (at least in the vicinity of solid surfaces), nanotechnology applies directed transport processes— such as migration in an electrical field in a liquid phase.

Cold plasma, ion or neutral particle beams are well—suited to the direct­ed transport of particles onto solid surfaces. They utilize electrical fields to force charged or secondary neutralized particles in a given direction.

Adsorption and binding equilibrium are ob­served on micro structured substrates, and also sometimes on nanostructured surfaces. Therefore, forward and reverse reactions have to exhibit a certain probability in a given time range. This process can be described by equilibrium constant K_ad (binding constant), e.g. for the binding of particles onto surfaces:

Classical chemical kinetics based molecu­lar statistics are not applicable to processes involving a small number of particles. This is the case for the situation in a chemical equi­librium. For micro structured binding spots known from biochips with several million similar molecules, the effect can even be ob­served: a chip of 3 µm² for an assumed foot­print of 0.3 nm² per molecule exhibits about 107 binding places.

With a binding constant of 10⁶, on average, 9999 990 places are occupied, but 10 remain free. So a prerequisite for the statistics of large numbers is not given. For a reduced binding area of 300 nm² the statistical method fails even for binding constants below 10³.

Also, in the classic case, reactions with con­ditions close to equilibrium are driven by probability in nanotechnology. In addition to the external electronic, physical and chemi­cal factors, a molecular quantum noise can be observed and reduces the signal to noise ratio. This phenomenon is of particular im­portance for operations with large libraries of substances, as, for example, in the case of DNA computing.

Molecular operations in nanotechnol­ogy should be considered under the aspect of a certain distance from the equilibrium. So nanotechnology can be compared with bio-molecular morphogenesis, where essen­tial processes also take place away from the equilibrium.

The kinetic trick in nature is the application of fast reactions with small participating particles (e.g. equilibrium in protonation or the creation of coordinated compounds) to support slow processes away from the equilibrium, such as the construc­tion of super-molecules or subcellular struc­tures. Hence enzymatic processes become important in nanotechnology.

In the classical chemical sense, enzymes are catalysts, and the enhanced reactions are usually close to equilibrium. For the same number of similar particles this view is identical to inorganic and non-biogenic organic catalyst.

However, for small number s of particles and the appli­cation of a probability approach, the question arises as to whether the enzyme is activated after coming into contact with a substrate molecule. Depending on the activation of the individual catalyst molecule, the substrate is preserved or converted into the product. This behaviour is similar to a classical tool. Because of the probability conditions, and the level of small particles, catalysts are tools rather than reaction partners.