Nano Chemistry

Nanochemistry is concerned with generating and altering chemical systems, which develop special and often new effects as a result of the laws of the nanoworld. The bases for these are chemically active nanometric units such as supramolecules or nanocrystals. Nanochemistry looks set to make a great deal of progress for a large number of industry sectors.

Nanotechnology exists in the realm where many scientific disciplines meet. Achievements in physics are getting progressively smaller – from valves to electronics, down to microelectronics and quantum computing. It mirrors the downsizing in focus in the biological sciences, from cells to genomics. Conversely, achievements in chemistry have been converging into the nanometre range from below – from atoms and molecules to supramolecular chemistry. Nanochemisty focuses on the unique properties of materials in the 1–100 nm scale. The physical, chemical, electrical, optical and magnetic properties of these materialsare all significantly different from both the properties of the individual building blocks (individual atoms or molecules), and also from the bulk materials.Nanochemistry is a truly multidisciplinary field, forming a bridge between nanotechnology and biotechnology, spanning the physical and life sciences.

The Nanochemistry Research Institute (NRI) at Curtin carries out world-class research to provide innovative solutions to 

-energy and resources

-materials and manufacturing

-electronics

-agricultural

-environmental management, and

-health and medical industries

Nanochemistry applications in the materials, resources and energy sectors range from the design of crystalline catalysts and the control of crystal size, morphology, phase and purity, to the design and use of additives to control crystallization and inhibit scale formation. In the biological field, control of chemistry at the supramolecular level can lead to the development of a wide variety of new and improved biomaterials, such as artificial bones and tissues, as well as new pharmaceuticals and improved methods of drug delivery.1

‘‘We are like dwarfs on the shoulders of giants, so that we can see more than they.’’  

Bernard of Chartres, 12th century with nanoscience being the discipline concerned with making, manipulating and imaging materials having at least one spatial dimension in the size range 1–1000 nm and nanotechnology being a device or machine, product or process, based upon individual or multiple integrated nanoscale components, then what is nanochemistry? In its broadest terms, the de.ning feature of nanochemistry is the utilization of synthetic chemistry to make nanoscale building blocks of different size and shape, composition and surface structure, charge and functionality. These building blocks may be useful in their own right. Or in a self-assembly construction process, spontaneous, directed by templates or guided by chemically or lithographically de.ned surface patterns, they may form architectures that perform an intelligent function and portend a particular use.2

-Creating nanoparticles

-Allowing properties of nanosystems to evolve, manipulating and controlling  them Encapsulating and transporting materials (e.g. deodorant with nanodroplets)4

1.3Nanochemistry used in: -

Cosmetics, e.g. sunscreen, toothpaste, skincare products

Sanitary ware

Built-in ovens and baking trays

Gas-tight packaging

Screens, photographic films

Separating technology for waste water treatment and food production

Catalysers for chemical reactions

Exhaust purification5

It is also used in formation of :-

Commercialization of nanochemicals

Nanooxides of precious, ferromagnetic, rare metals (Ti, Zr etc.)

Nanopolymers and membranes

Nanomaterials (cement, fertilizers

Nanopowders in chemical applications

Nanogreen chemistry

Nano energy applications

Environmental applications of nanotechnology

When thinking about self-assembly of a targeted structure from the spontaneous organization of building blocks with dimensions that are beyond the sub-nanometer scale of most molecules or macromolecules, there are five prominent principles that need to be taken into consideration. 

These are: (i) building blocks, scale, shape, surface structure, (ii) attractive and repulsive interactions between building blocks, equilibrium separation, (iii) reversible association–dissociation and/or adaptable motion of building blocks in assembly, lowest energy structure, (iv) building block interactions with solvents, interfaces, templates, (v) building-blocks dynamics, mass transport and agitation. 

A challenge for perfecting structures made by this kind of self-assembly chemistry is to .nd ways of synthesizing (bottom-up) or fabricating (top-down) building blocks not only with the right composition but also having the same size and shape. No matter which way building blocks are made they are never truly monodisperse, nless they happen to be single atoms or molecules. There always exists a degree of polydispersity in their size and shape, which is manifest in the achievable degree of structural perfection of the assembly and the nature and population of defects in the assembled system. Equally demanding is to make building blocks with a particular surface structure, charge and functionality. Surface properties will control the interactions between building blocks as well as with their environment, which ultimately determines the geometry and distances at which building blocks come to equilibrium in a self-assembled system. Relative motion between building blocks facilitates collisions between them, whilst energetically allowed aggregation deaggregation processes and corrective movements of the self-assembled structure will allow it to attain the most stable form.6 

Providing the building blocks are not too strongly bound in the assembly it will be able to adjust to an orderly structure. If on the other hand the building blocks in the assembly are too strongly interacting, they will be unable to adjust their relative positions within the assembly and a less 1 ordered structure will result. Dynamic effects involving building blocks and assemblies can occur in the liquid phase, at an air/liquid or liquid/liquid interface, on the surface of a substrate or within a template co-assembly. As this text describes, building blocks can be made out of most known organic, inorganic, polymeric, and hybrid materials. Creative ways of making spheres and cubes, sheets and discs, wires and tubes, rings and spirals, with nm to cm dimensions, abound in the materials self-assembly literature. They provide the basic construction modules for materials self-assembly over all scales, a new way of synthesizing electronic, optical, photonic, magnetic materials with hierarchical structures and complex form, which is the central theme running throughout this chapter. A .owchart describing these main ideas is shown in Figure 1.

 

Figure 1.  A fiowchart delineating the factors that must be considered when approaching the self-assembly of a nanoscale system

Nano-, a pre.x denoting a factor, its origin in the Greek nanos, meaning dwarf. The term is often associated with the time interval of a nanosecond, a billionth of a second, and the length scale of a nanometer, a billionth of a meter or 10 A ° . In its broadest terms, nanoscience and nanotechnology congers up visions of making, imaging, manipulating and utilizing things really small. Feynman’s  prescient nano world ‘‘on the head of a pin’’ inspires scientists and technologists to venture into this uncharted nano-terrain to do something big with something small.7 

It was not so long ago in the world of molecules and materials that 1 nm (1 nm ¼ 10 A ° ) was considered large in chemistry while 1 m m (1 m m ¼ 1000 nm ¼ 10,000 A ° ) was considered small in engineering physics. Matter residing in the ‘‘fuzzy interface’’ between these large and small extremes of length scales emerged as the science of nanoscale materials and has grown into one of the most exciting and vibrant fields of endeavor, showing all the signs of having a revolutionary impact on materials as we know them today. In our time, ‘‘nano’’ has left the science reservation and entered the industrial technology consciousness and public and political perception. Indeed, bulk materials can be remodeled through bottom-up synthetic chemistry and top-down engineering physics strategies as nanomaterials in two main ways, the first by reducing one or more of their physical dimensions to the nanoscale and the second by providing them with nanoscale porosity. When talking about finely divided and porous forms of nanostructured matter, it is found that ‘‘nanomaterials characteristically exhibits physical and chemical properties different from the bulk as a consequence of having at least one spatial dimension in the size range of 1–1000 nm’’. 

2. NANOCHEMISTRY INTERRELETED WITH NANOLITHOGRAPHY                                 

The emergence of nanoscience and nanotechnology depends, at least partially, on the ability to position, manipulate and fabricate a variety of structures, materials and devices with accuracy in the nanometer-scale. It also rests on the incorporation of organic and biological molecules as active components of mechanical, electrical or molecular recognition devices. Furthermore, for progress to be made in this area it is necessary to understand the underlying science and engineering involving nanoscale structures and devices, which experimentally requires, among other things, the development of accessible, flexible and robust nanofabrication approaches. Conventional nanolithographies, i.e., those derived from optical and electron beam lithographies are either cost-intensive or unsuitable to handle the large variety of organic and biological systems available in nanotechnology. The above driving forces have stimulated the development of alternative nanofabrication techniques. This process started approximately in 1990 and it has given rise to the establishment of three major nanolithography methods:

nano-imprint lithography, 

soft lithography and

scanning probe-based nanolithographies(SPL). 

The development of nanometer-scale lithographies is the focus of an intense research activity because progress on nanotechnology depends on the capability to fabricate, position and interconnect nanometer-scale structures. The unique imaging and manipulation properties of atomic force microscopes have prompted the emergence of several scanning probe-based nanolithographies. In this tutorial review we present the most promising probe-based nanolithographies that are based on the spatial confinement of a chemical reaction within a nanometer-size region of the sample surface. The potential of local chemical nanolithography in nanometer-scale science and technology is illustrated by describing a range of applications such as the fabrication of conjugated molecular wires, optical microlenses, complex quantum devices or tailored chemical surfaces for controlling biorecognition processes.8

Scanning probe microscopes, in particular atomic force microscopes (AFM), enjoy a prominent status in nanotechnology because of their ability to image at sub-10 nm resolution a wide variety of surfaces ranging from biomolecules to integrated circuits. Furthermore, the performance of the instrument is not compromised by the medium. High resolution imaging has been achieved in air, liquids or vacuum. The relative ease of conversion of a force microscope into a modification tool has prompted a fascinating variety of atomic and nanometer-scale modification approaches. Those approaches involve the interaction of a sharp probe with a local region of the sample surface. Mechanical, thermal, electrostatic and chemical interactions, or several combinations among them, are currently exploited to modify surfaces at the nanoscale with probe microscopes.9 

Among the most successful scanning probe nanolithographies (SPL) are those based on the spatial confinement of a chemical reaction within a nanometer-size region. The region is usually defined by either a combination of the probe-sample surface geometry or the combination of the geometry and another factor that could be an external electrical field or a liquid meniscus. In particular nanometer-size water meniscus have appeared as remarkable tools to either confine the lateral extension of a chemical reaction or to control the lateral diffusion of organic molecules. Given the meniscus nanoscale size, nanofabrication and nanochemistry are intimately linked in the SPL methods reviewed here.10

This tutorial review aims to provide an overview of scanning probe nanolithography methods where the nanofabrication process is mediated by a local chemical reaction. In particular we discuss local oxidation nanolithography and its generalization through the use of polar and non-polar liquids as well as other emerging methods such as dip-pen nanolithography and chemomechanical patterning. Those methods allow exploration of the relationship existing between nanolithography and nanochemistry. They could be integrated under the name of local chemical nanolithographies. Some of the probe-based methods are still in their infancy, i.e., the emphasis is placed on the (nano)patterning of a given region of the sample surface while others such as local oxidation have reached a higher level where fabrication and device performance are strongly emphasized. Nonetheless, all of them are in a strong development stage.

In general, local chemical nanolithographies offer to the academic researcher a low-cost and versatile alternative to fabricate and investigate the properties of a large variety of organic and inorganic structures, and devices at the nano-scale. The flexibility to handle organic, biological and inorganic surfaces alike provided by local chemical nanolithographies may explain their growing acceptance in the nanoscience and nanotechnology community11

Since the inauspicious discovery of scanning probe-based oxidation in the heyday of atomic-scale manipulation of surface by scanning tunnelling microscopy (STM), local oxidation nanolithography has evolved to become a useful tool to fabricate sophisticated devices for studying a variety of quantum phenomena such as coulomb blockade, quantum conductance and the like. In 1990 Dagata and co-workers modified a hydrogen-terminated silicon surface by the application of a bias voltage between an STM tip and the surface. Secondary ion mass spectroscopy showed the presence of silicon oxide in the modified regions. In 1993 it was demonstrated that local oxidation experiments could be performed with an atomic force microscope. This observation paved the way for the development and expansion of local oxidation approaches to modify surfaces. The versatility of the method together with the astonishing variety of materials amenable for anodic oxidation explains the wide use of this scanning probe-based nanolithography. Local oxidation nanolithography (LON) is sometimes called scanning probe oxidation, nano-oxidation, local anodic oxidation or generically AFM lithography.12

The present knowledge allows some similarities between local oxidation and conventional anodic oxidation to be established. The AFM tip is used as a cathode and the water meniscus formed between tip and surface provides the electrolyte. The meniscus is usually induced by the application of an electrical field, although in some cases it can also be driven by the mechanical contact between tip and sample surface. The end result is the formation of a nanometer-size electrochemical cell (nanocell) that contains about 5 × 104 molecules. The method to form liquid bridges is so precise that water meniscus diameters of 20 nm are easily obtained. This has lead to the reproducible fabrication of sub-10 nm structures in Si and even smaller structures in titanium films.

 Fig2.1(a) Schematics of a local oxidation nanolithography experiment. The meniscus provides the oxyanions and confines the spatial extent of the reaction. (b) Accepted chemical reactions in the local oxidation of a metallic surface.

Local oxidation experiments were first performed on Si(111) and polycrystalline tantalum faces. Since then a large number of materials have been locally oxidized such as compound III–V semiconductors, silicon carbide, several metals such as titanium, tantalum, aluminium, molybdenum, nickel and niobium; perovskite manganite thin films, dielectrics such as silicon nitride films as well as organosilane self assembled monolayers, dendritic objects and carbonaceous films.

Silicon faces either with or without hydrogen passivation have been thoroughly modified by LON. They serve as a good model to illustrate some of the fundamental aspects involved in the local oxidation process. Voltage pulses are applied to generate an oxide dot. The dot size depends linearly on voltage strength but the dot height shows a power law dependence of the type 

 h  (t/t0) 

(1)

where  is in the 0.1–0.3 range. Voltage pulses usually change from experiment to experiment but they are in 10–30 V and 0.005–1 s range respectively. 

The local oxidation process is accompanied by an extremely small current. The value depends on the final dot size. Common values are in the sub-picoampere regime. Experimental observations have shown that the current through the interface during oxidation matched the current calculated from the measured oxide volume by taken into account that four elementary charges are needed to oxidize one Si atom. This observation together with the presence of a water meniscus points out the electrochemical nature of the process.

For silicon, the reactions in the nanocell are described by the following half-cell reactions. In the anode (sample surface) the oxidation takes place according to 

 Si + 2h+ + 2(OH–)  Si(OH)2  SiO2 + 2H+ + 2e–(2)

While hydrogen generation occurs at cathode to complete the electrochemical reaction 

 2H+(aq) + 2e–  H2

(3)

For a metallic surface the following half-cell reaction (anode) has been proposed, 

 M + nH2O  MOn + 2nH+ + 2ne–

(4)

Photoemission spectroscopy on Si and GaAs have confirmed that indeed the chemical composition of the fabricated structures were silicon and gallium arsenide oxides respectively. Usually heights are measured with respect to the substrate baseline, consequently reported oxide heights differs from the true oxide thickness. In Si approximately 60% of the oxide is above the substrate baseline.

A feature that distinguishes local oxidation nanolithography from other scanning probe-based modification methods is that modification and imaging processes operate independently. Long range van der Waals forces and/or short range repulsive forces are used to image the surface while an external voltage is applied to induce the local oxidation of the surface. Another relevant feature is that it allows in situ control of the device electrical and topographic characteristics during its fabrication.

Local oxidation processes allow the direct fabrication of three elements, dielectric barriers, masks for selective etching and templates. The combination of those elements allows the fabrication of a wide range of electronic and mechanical devices with nanometer-scale features. In some cases LON is used in combination with other methods such as photolithography, electron beam lithography or chemical wet etching to fabricate the desired device. In those cases, the critical or most relevant features of the device are fabricated by local oxidation. The list of devices fabricated by LON is quite large, it includes data storage memories, conducting nanowires, side-gated field-effect transistors, single electron transistors, superconducting quantum interference devices, quantum points and rings, microlens, templates for the growth of biomolecules and conjugated materials and etch resistant masks.

The flexibility of LON to pattern arbitrarily shaped patterns is illustrated in. Alternating insulating and semiconducting rings, arrays of dots and the first ten lines of  Don Quixote  have been written by local oxidation.

Although one of the strengths of LON resides in the unique imaging and positioning properties of AFM, recent experiments have demonstrated that the fundamental physical and chemical processes governing local oxidation are scalable. Millimeter square regions have been patterned in a few seconds by using stamps with millions of nanometer-size protrusions, each of them acting as single AFM tip. Those experiments open new routes for bridging nano and macroscale devices.13

At this point it seems rather straightforward to pose oneself the question of what would happen if other liquids are used. Would it be possible to form reproducible patterns on the sample surface? Would it be possible to change the chemical composition of the formed structures? Tello and Garcia have performed modification experiments with polarizable organic solvents such as ethyl and propyl alcohols. In the case of the experiments performed with ethanol menisci, photoemission spectroscopy analysis revealed that the fabricated structures on a silicon surface were no longer oxides but showed the formation of silicon carbide and carbon sp2 compounds. 

Frechet and co-workers have shown that operating the AFM in a liquid cell filled with n-octane gave rise to the formation of nanostructures that could be used as etch resistant resists. Because the fabricated structures were not etched by HF, it was concluded that those structures were not silicon oxides. However, experiments performed in other non-polar organic solvents such as hexadecane and 1-octene gave rise to structures that showed chemical and kinetic behaviour consistent with field induced oxidation in air. The above experiments reveal that on the same substrate (Si) the composition of the fabricated structure is organic solvent dependent.

Another interesting experiment that emphasized the electrochemical character of the modification process was performed by Li and co-workers. A silicon tip was coated with a water-soluble metal salt. The formation of a water meniscus dissolves some little amount of salts. When a positive voltage is applied, the metal ions dissolved in the water meniscus are reduced and deposited as a metal particles on the cathode. The method was applied to form Au, Ge, Ag, Cu and Pd structures.

Successful AFM modification experiments have also been performed in inert atmosphere to avoid water meniscus formation. For example, the localized chemical activation of a protected amine surface by the application of a voltage bias between the AFM and the silicon substrate has been reported. 

All the above experiments were performed with rather similar experimental set ups, i.e., an AFM with a voltage biased tip-sample interface enclosed in an environment control chamber. Consequently, the different modification methods discussed so far could be considered variations of a single probe nanolithography based on the spatial confinement and field-induced generation of chemical reactions. The spatial confinement could be achieved either by the formation of a liquid meniscus (nanocell) or by focusing the electrical field lines underneath the tip. In this context it fits to describe all of the above methods under the single term of local chemical nanolithography.15

In 1995 Jaschke and Butt reported the deposition of organic molecules (octadecanethiols) from an AFM tip onto a mica surface. This process was rediscovered and developed for nanopatterning purposes by Mirkin and co-workers. The method was called Dip-pen nanolithography (DPN). In DPN inks are first adsorbed on the tip of an AFM, then transferred by diffusion to a pre-selected position of the substrate. The first DPN experiment was demonstrated by patterning alkanethiol self-assembled monolayers onto gold surfaces. The combination of the flexible terminal group chemistry given by alkylthiols and the AFM nanoscale control over physical dimensions explains the remarkable expectations raised by dip-pen nanolithography. The DPN approach is illustrated in Fig. 2.2

 Fig. 2.2 Scheme of the dip-pen nanolithography ink deposition.

Some proteins and DNA molecules are amenable for thiol functionalization, thus they are also suitable for DPN. Patterns of DNA have been written on gold surfaces using hexanethiol-modified nucleotides. Furthermore, the resulting patterns were hybridized to DNA-functionalized gold nanoparticles. Proteins patterns have also been formed by DPN either by using thiolate proteins or by direct deposition of proteins on pre-prepared glass surfaces. The array of retronectin proteins depicted in Fig 2.3 shows potential of DPN for studying and controlling biorecognition processes.

 Fig.2 .3 AFM image of a retronectin protein array patterned by DPN

Until now few substrates have been suitable for DPN, they include gold, silicon, silicon oxide and glass, however, the list of inks successfully deposited by DPN is growing. They include alkylthioles, organosilanes, proteins, DNA, organic dyes, dendrimers and sols among others. The minimum feature size achieved by dip-pen is 15 nm for alkathiols inks on single-crystal gold surfaces.

The fundamental processes governing the transfer of organic molecules from the tip to the sample surface are still under debate. Mirkin et al. hypothesized that the diffusion process was mediated by the presence of a water meniscus. The formation of a water meniscus is an unavoidable fact in contact AFM operation in ambient conditions. However, other authors have argued that hydrocarbons are essentially insoluble molecules in water, so it seems unlikely that they could be transferred via the water meniscus. Furthermore, they succeeded in performing DPN experiments with octadecanethiol molecules on gold surfaces at 0% relative humidity.

A remarkable observation of the diffusion process is that all the deposited molecules obey the same functional form with respect to the time, area = kt + b, where k depends on ink, temperature and in some cases humidity while b reflects the tip size/coating dependence. The above expression underlines some of the limitations of DPN. For practical reasons, imaging and deposition are performed with the same tip, thus inked tips inherently cause contamination on the surface. Another limitation is the little control of deposition rate once the molecules have been adsorbed on the tip.

Dip-pen nanolithography has also been performed with arrays of cantilevers which opens the possibility for high-throughput parallel approaches.15

Exerting mechanical forces by the tip is the most intuitive approach to modify a surface by an AFM. Consequently in the late 80s and early 90s many research groups reported results on scratching surfaces by AFM. However, most of those approaches have been abandoned because of poor reproducibility and tip degradation. Usually the tip is irreversible damaged (blunted) after a few modifications. Nonetheless, several groups have recently revisited this approach to patterning of surfaces. The renewed interest in using mechanical forces comes from a combination of factors. On one side, state-of-the-art AFM technology allows the modulation of forces with nN accuracy. On the other side, the use of soft layers such as self-assembled alkyl thiol monolayers as sacrificial layers allows material removal without tip degradation. Usually mechanical removal is accompanied by the deposition of selected molecules that form covalent bonds with the exposed surface, termed chemomechanical patterning.

Chemomechanical patterning has the following experimental protocol. First, it requires the definition of two range of forces. Low forces, say of about 1 nN for imaging and high forces, say of about 10–50 nN for modification. Each sample has its set of threshold values. The surface to be modified either a self-assembled monolayer or a rigid substrate and the AFM tip are immersed in a fluid cell. At low forces the AFM images the surface. By increasing the force above a certain threshold, the SAM becomes disordered and adsorbates are displaced leaving the bare substrate. This is usually called nanoshaving. If the removal of the sacrificial SAM is done in the presence of another molecule solution with a higher concentration than the displaced SAM, the new molecules will be grafted onto the substrate surface. High-density alternating nanostructures of octodecanethiol and decanethiol with a periodicity of 14 nm have been fabricated on gold surfaces. If the shaving of the SAM is performed in a solution containing gold nanoparticles, the nanoparticles then adsorb onto the exposed areas.16 

Fig.2.4 Scheme of chemomechanical patterning. (a) A SAM is assembled on the surface. (b) The AFM tip exerts a force on the SAM and removes the monolayer in a certain region (nanoshaving). (c) A different monolayer can be self-assembled in the swept region (nanografting).

Local removal of SAMs with an STM have been practised for several years. Gorman et al. have combined the local removal of a SAM with its replacement with a second thiol. SAMs composed of n-alkanethiol on Au(111) were locally removed by increasing the voltage bias. The removal was performed in a fluid solvating a second replacement thiol. The first demonstration of this method was provided by generating n-alkanethiolate SAM patterns composed of regions of different chain lengths. The only fundamental different between chemomechanical patterning and replacement is the interaction that produces the substitution, mechanical forces or voltage-induced processes respectively.17

The influence of molecular architecture on the properties and behavior of small molecules has become one of the pillars of small molecule chemistry, (i.e. geometric/positional isomerism, optical stereoisomerism, tautomerism, etc). Such analogous architectural isomerism has been noted recently for property differences observed for the five known forms of carbon, involving structures that transcend from the nanoscale to the macro-scale level. More recently, the importance of “macromolecular isomerism” has been recognized . Prior to 1984 , only three synthetic polymer architectures were known, namely; (I) linear (II) cross-linked and (III) branched type configurations. These architectural discoveries have been characterized by the emergence of new syntheses, structures, phenomena, properties and products that have dramatically improved the human condition during this past century.

The “dendritic state” is a new, fourth class of polymer architecture) consisting of four subclasses: (a) random hyperbranched polymers, (b) dendrigrafts, (c) dendrons, and (d) dendrimers. The monodisperse nature of dendrons and dendrimers makes them important for nanoscientists. They are unlike traditional polymers in that critical nanoscale parameters, such as size, shape, and presentation of chemical functionality, can be precisely controlled through their architecture (i.e., their cores, interiors, and surfaces). The core may be thought of as the molecular information center. It determines the size, shape, directionality, and multiplicity of surface functionality. Within the interior, the branch cell amplification region is found. This defines the volume and type of containment space enclosed by the terminal groups, offering a variety of possible “guest-host relationships”. Finally, the surface consists of reactive or passive terminal groups. These may serve as polyvalent nanoscaffolding, upon which new generations of dendrimers can be covalently attached for further growth. Alternatively, the surface groups may function as control gates for the entry and departure of guest molecules from the interior . The core, interior, and surface determine all the properties of dendrimers. 

With the exception of biological polymers, or perhaps fullerenes, no other covalent structure offers such ‘bottomup’ control. Precise dendritic synthesis strategies (i.e. divergent/convergent) for producing over 100 reported dendrimer/ dendron compositional families possessing over 1000 different surface /interior chemistries have been reported . These dendritic entities have been used as simple nanodevices or as nanoscale building blocks for the synthesis of more complex nano structures . Certain statistical dendritic polymers (i.e., random hyperbranched) are now viewed as a penultimate topology in the continuum of architectures that reside between the two classical areas of “thermoplastic” and “thermoset” polymers. Thermoset polymer pioneers such as Dusek, et. Al conclude that “random hyperbranched polymers” best represent the critical, penultimate thermoplastic architectural precursors that lead to the traditional thermoset state. In contrast, the quantized precision of dendrons/dendrimers allows these entities to be viewed as nanoscale monomer type building blocks, suitable for the construction of regio-cross-linked dendrimers referred to as; “megamers”.

Polymer history has shown that independent of elemental composition, each new atom reconfiguration leading to the original three traditional architectural types has produced completely new and unexpected properties. This concept has validated the acceptance of “dendritic polymers” as the newest and fourth major class of macromolecular architecture. This is based on the fact that entirely new properties/behaviors, unprecedented in traditional polymer architectures, are clearly manifested by this macromolecular class . Since ordinary monomeric building blocks are used in the construction of these dendritic architectures, one cannot claim that new dendritic properties are predictable by simple extrapolation of the building block properties. A sampling of some well recognized dendritic effects, relative to classical linear polymer architectures.18 

In the past five years, worldwide nanotechnology initiatives have created an international focus on new synthesis strategies, structures, phenomena, and properties associated with dimensional length scales residing between 1-100nm. These dimensions encompass those associated with many key biological building blocks (i.e., life sciences; protein, DNA, RNA, etc.), as well as nano dimensions in the electromagnetic energy spectrum (i.e. X- ray, UV) relating to critical abiotic application areas of interest (i.e. advanced materials; nano-photonics, nano-electronics).

Presently, an international focus is emerging on “nanotechnology.” It has been described as the “ultimate scientific frontier” that will both define and lead the world into the next industrial revolution. New nanophysical/ chemical properties related to precisely defined nano dimensions, shapes and functional group presentations are being reported on a regular basis. A critical key to advancing progress in synthetic nanochemistry/technology will dependent largely upon identifying appropriate quantized, nanoscale building blocks, much as were required for the development of the traditional chemistry (i.e.,periodic elements) and polymer fields (i.e., monomers) . The challenge is to develop critical structure-controlled methodologies to produce well defined nanoscale modules that will allow cost-effective synthesis and controlled assembly of more complex nanostructures in a very routine manner. Such nanostructures will be macromolecular, require the controlled assembly of as many as 103 - 109 atoms and possess molecular weights ranging from 104 – 1010 Daltons.

Nature solved these problems and shattered this nanoscale synthesis barrier several billions of years ago. These evolutionary events set the stage for nano dimensional scaling that today determines essentially all the significant molecular level structures and parameters dealing with life (i.e., proteins, DNA, RNA, etc.). These same parameters that include nanoscale sizes, nanosurfaces/ interfaces, nanocontainment, nanoscale transduction/amplification and information storage have important implications, not only in biology, but in critical abiotic areas such as catalysis, computer miniaturization, nano-tribology, sensors, and new materials. “Bottom-up” synthetic strategies that produce size-monodisperse, well-defined organic and inorganic nanostructures (dimensions ranging between 1-100 nm) will be of utmost importance. It is now well recognized, that dendritic strategies allow the systematic construction of nanoscale structures and devices with precise atom-by-atom control as a function of size, shape, and surface chemistry . The versatility of dendrons/dendrimers in such a role is becoming widely accepted. They are recognized as quantized nanoscale building blocks possessing precise nanometer sizes (nanoparticles), solvent filled interior void spaces for unimolecular encapsulation (nano-containers) and mathematically defined numbers of surface functionality (nano-scaffolding) .19

 

           Laser has high potential in advancing chemical research by realizing new spectroscopy, nalysis, reaction, and fabrication, while its spatial resolution was limited to light wavelength. Dr. Hiroshi Masuhara has utilized various lasers and microscopes, developed new spectroscopy and imaging methods with nm resolution, explored novel nm chemical phenomena, elucidated their mechanism and dynamics, and extended the studies to material and bio applications.    

 

Laser Nano Spectroscopy and Nano PhotochemistryDr. Masuhara and his colleagues developed various time-resolved reflectionspectroscopies and demonstrated how very essential they are for solid state photochemistry. They initiated and contributed by a series of seminal papers to the development of ps- and fs-diffuse reflectance, ps- and fs-regular reflectance, ps-total internal reflectance (evanescent wave absorption and fluorescence), and fs-transient grating spectroscopies. Photophysical and photochemical dynamics of nano crystals, nm-thin films and nm-surface/interface layers of solids were measured and analyzed with high energy and temporal resolutions similar to those in solution. Ultrafast intersystem crossing, charge separation, exciplex formation, and photothermal heating processes characteristic of the “solid” state were elucidated and first reported by the Masuhara group for aromatic molecules, dyes, EDA complexes, TiO2,polymers,andresists.

The single particle approach has been applied to understand spectroscopic properties and photophysics and photochemistry of nanoparticles. Spectroscopy of individual nanoparticles is examined as functions of their shape, size, morphology, internal structure, and environment. By combining an inverted optical microscope and AFM, the Masuhara group developed novel detection techniques such as Rayleigh light scattering spectroscopy of individual nanoparticles in addition to single particle fluorescence spectroscopy. Novel nm size effects on aromatic/polymer crystals and polymer nanospheres were investigated and their origins were confirmed to be structural confinement, namely, size-dependence of crystal lattice softening, molecular packing, and polymer conformation. These novel effects are characteristic of molecular material, and completely different from well-known nm size effects of semiconductors and metals which are due to electron confinement. This single nanoparticle spectroscopy is complementary to single molecule spectroscopy and can bridge the gap in understanding the relation between properties of molecules and materials.21

              A focused near-infrared laser beam exerts photon force on nano materials, enabling their manipulation. The optical trapping studies have been conducted by physicists, while its extension to chemistry was started by the Masuhara group. The minimum size of the trapped particles in solution at room temperature was confirmed to be a few nm. The force is strong enough to suppress electrostatic repulsion between electrolytes and to break hydrogen-bonding network around polymers in water. Larger photon force is exerted on molecules with higher polarizability. Such molecular structure-photon force relation opened a new field that could be coined “chemistry of photon force”.

               This is further being extended to trapping dynamics of single nano particles in solution at room temperature. For 100 nm-sized particles, successive trapping was followed one by one, while photon-force assisted-aggregation was clearly confirmed. Furthermore, absolute potential shape of an optical trapping-well was determined by measuring directly fluctuation of a single probing microparticle in solution, while spectroscopic identification of assembled structures in the trapping potential was done for model gold nanoparticles. By the 3-dimensional trapping and manipulation of nanoparticles and their fixation onto substrate, the nanoparticles were patterned with resolution of a few tens nm. Although the spatial resolution is less than that of the 2-dimensional surface manipulation by a STM tip at low temperature under vacuum, the much broader applicability is very important for biological material, cells, and protein crystals.23

              Intense pulsed laser excitation of nano droplets, aggregates, crystals, powders, and films generates high density excited states and intermediates. Their mutual interactions and their successive absorption of excitation photons lead to ablation, expansion/contraction, surface protrusion, and so on. The Masuhara group developed time-resolved imaging methods to probe laser-induced morphological dynamics and combined them with the time-resolved spectroscopy. Then they extended systematic studies on ns-, ps-, and fs-laser ablation and related phenomena and determined rates of expansion, surface roughening, ablation, and contraction. Namely, the Masuhara group demonstrated how electronic excitation of molecules in solids evolves leading to morphological changes. On the basis of these results, molecular mechanisms of laser ablation could be proposed: a cyclic multiphotonic absorption mechanism in the case of ns-excitation and for fs-excitation a transient pressure mechanism due to rapid photothermal conversion. All the processes are within the framework of a classic Jablonski diagram without additional states and intermediates such as plasma. Hence laser ablation and related dynamics are now understood as typical nonlinear photochemicalphenomena.23

                 By utilizing the transient pressure induced by fs-excitation, Masuhara and his colleagues unraveled novel laser ablation phenomena and developed new methodologies. Femtosecond multiphoton excitation of molecular films gives discrete and multistep etching, fs ablation of microcrystals in solution gives the smallest nanocrystal of a dye with a size of 13 nm as a stable nanocolloid, and a shockwave induced by the transient pressure can remove a living cell from a substrate without damage. Recently the In addition, they succeeded in controlling the crystal growth with fs-multiphoton excitation. These results have important implications in the area of bioscience.

 As described above, Dr. Masuhara has proposed and demonstrated new methodologies and concepts of nano chemistry which can be realized only by laser. His scientific achievement has a great impact not only on chemistry but also on physics, nano material engineering, and life science, and is opening frontier in photoscience. It has been recognized widely and internationally, and is enough eligible for the Chemical Society of Japan Award.24

Research in nanochemistry demands a multidisciplinary approach. Projects can involve bulk synthetic techniques, computational molecular modelling, advanced microscopy techniques, even the synthesis of new molecules designed to spontaneously assemble into complex architectures. This is reflected in the Institute’s three core areas of expertise. The molecular simulation/computational chemistry/molecular modelling research group is one of the strongest in Australia, and is recognized worldwide. The solid theoretical basis is used to understand and visualise what is occurring at the nanoscale.

Nanocharacterisation expertise, encompassing scanning probe, microscopy and analytical facilities, enables the real-time observation of nanoparticulate growth processes.

Expertise in nanoreactions extends to the use of nano-containers, for molecular recognition, chemical storage, or to contain reactions on an extremely small scale. Capabilities in supramolecular and synthetic chemistry ensure that target compounds can be tailored to a specific size, shape and functionality, for particular applications, such as additives for crystal growth modification, scale mitigation, or product improvement.26

5.1 New Directions of nanochemistry in Nanoscience and Nanotechnology 

With the coming of nano-revolution, the nano-inspired researches have been popular throughout the world. The nanoscience and nanotechnology have taken much of the attentions from thousands of scientists and engineers. These specialists are struggling  to deal with the fantastic intellectual challenges in the new-born areas, such as nanophysics, nanochemistry, nanomechanics, nanomaterials, nanometrology, nanooptics, nanoelectronics, nanomedecine and bio-nanotechnology.  The report from the Royal Society of London entitled ‘Nanoscience and  Nanotechnology: Opportunities and Uncertainties’was published on 29 July 2004. 

This report highlighted the significance of the nanoscale, and addressed the  uncertainties about the health and environmental effects of nano-particles. It also gave the consensual definitions of nanoscience and nanotechnology. Because these definitions are rather vague, many scientists immediately look forward to an exact and scientific definition of the term “nano”. A proud history of exploring nature is partly consisted of the activities for the manipulation of a molecule or a number of atoms via sufficiently precise micro-instruments. In present, many scientists are devoted to further knowing of the naturally occurring molecular assemblies that regulate and control biological systems. By using ‘bottom-up’ nano-fabrication techniques, engineers long for producing the bio-materials and devices including molecular biomimetics system, molecular motors, unimolecular robots and bio-chip. Since numerous new-types of nanomaterials are manufactured and investigated, their remarkable properties are being understood gradually. The nanomaterials may realize our dream of finding materials with characteristics, functions and applications at smaller and smaller scales. Combining nanomaterials and information-communication technologies, technologists are able to gain a series of progresses in information storage, sensor, computer and ‘off-roadmap’technologies, and NBIC technology.26 

Each research area within the Institute spans both the physical and biological disciplines, and has applications in both the resources sector and life sciences.

Crystallization

Biomimetic Chemistry

Carbon Technologies

Nanoparticles

Molecular Recognition/Molecular Templating

Self-assembling Structures

Research focuses on the fundamental processes that underlie and ultimately control the production of crystals in an industrial plant. Factors governing properties such as the external shape or morphology of a crystal, rate of growth, crystal size and purity are being investigated. Research is also aimed at gaining a fundamental understanding of the chemistry and hydrodynamics of supersaturated solutions that hinder the formation of crystalline scale deposits. Scale formation is a serious industry-wide problem, and both engineering and chemical strategies to mitigate scale formation are under development.27

Developments made in the control of scale growth are now being applied to the study of calcium carbonate crystallization, and pathogenic biomineralisation such as kidney stones. Biochemical studies on the crystallization of lactose are also in progress, to improve lactose quality and increase growth rates in industrial lactose crystallizers, for applications in the dairy industry. The conversion of iron hydroxyoxides into goethite is being studied as a means of making iron precipitates more filterable and easy to handle in industrial systems. The same conversion may have applications in the treatment of iron overload diseases, as goethite is not toxic in biological systems.29

Carbon fullerenes and nanotubes possess properties that are at times very different to the atomic or bulk properties of carbon. The NRI aims to further the exploitation of  fullerenes in nanotechnology and biomedical technology, as components of novel pharmaceutical agents, including radiofullerenes, and in the production of nanodiamond films, for application in urable and low friction prostheses.29

Nanoparticles have resource applications ranging from catalysis to impurity removal. Nanoparticles of metals on base support are classical reagents for heterogeneous catalysis, and are fundamental to a number of industrial processes. As an example, efforts are now underway in the novel production of nanoparticulate gold for its unique catalytic properties. Biological applications of nanoparticles include inhalants and drug encapsulation technology for targeted or sustained release. Super-paramagnetic iron oxide nanoparticle research is bridging the life and physical sciences, finding applications both in tumour treatment and in high density information storage.30

Molecules able to selectively recognise and bind to solution species or specific surfaces have a myriad of applications. A number of additives have been developed that stabilise specific crystal faces, modifying crystal growth morphology, with application in scale mitigation and product formulation and purity.45

Whilst significant success has been achieved in miniaturising structures (for example, in microelectronics) through chemical and mechanical machining of bulk material (a top-down approach), it is increasingly clear that to advance into nano-dimensional structures, controlled assembly of building blocks (a bottom-up approach) is required. Research building on core strengths in crystallization and molecular recognition is leading to novel ways of building one, two and three- dimensional nanostructures. Research projects in these areas and many others are available for postgraduate student research. Industry needs in the area of nanochemistry can be addressed via service provision (particularly for material characterization or target synthesis), short term research contracts, or support of longer-term applied postgraduate research projects.

In recent years nanoscale science and technology have grown rapidly. Nanochemistry, in particular, presents a unique approach to building devices with a molecular-scale precision. One can envision the advantages of nanodevices in medicine, computing, scientific exploration, and electronics, where nanochemistry offers the promise of building objects atom by atom. The main challenges to full utilization of nanochemistry center on understanding new rules of behavior, because nanoscale systems lie at the threshold between classical and quantum behavior and exhibit behaviors that do not exist in larger devices.

Although nanochemical control was proposed decades ago, it was only recently that many of the tools necessary for studying the nanoworld were developed. These include the scanning tunneling microscope (STM), atomic force microscope (AFM), high resolution scanning and transmission electron microscopies, x rays, ion and electron beam probes, and new methods for nanofabrication and lithography.

Studies of nanochemical systems span many areas, from the study of the interactions of individual atoms and how to manipulate them, how to control chemical reactions at an atomic level, to the study of larger molecular assemblies, such as dendrimers, clusters, and polymers. From studies of assemblies, significant new structures—such as nanotubes, nanowires, three-dimensional molecular assemblies, and lab-on-a-chip devices for separations and biological research—have been developed.36

There are a great deal of issues which emerge with the development of nanoscience and nanotechnology. Some issues are directly related to nanomechanics, and result in  the following research topics: (1) mechanical properties and behaviors of  nanocomposite, nanowire and nanotube; (2) plastic deformation and fracture of  nanocrystalline metals; (3)size effect, surface effect and quantum effect at the  nanoscale; (4) multiscale simulations bridging spatial and temporal scales; (5)  nanotechnologies involve with biology, such as molecular biomimetics, protein  sidechain dynamics, bio-MEMS, bio-composite; (6) contact and adhesion mechanics  at the nanoscale including nano-wear and nano-tribology. 

The ultimate frontier of nanochemistry is the chemical manipulation of individual atoms. Using the STM, single atoms have been assembled into larger structures, and researchers have observed chemical reactions between two atoms on a surface. The use of atoms as building blocks opens new routes to novel materials and offers the ability to create the smallest features possible in integrated circuits (IC) and to explore areas like quantum computing. Until now the ever-decreasing size of IC circuitry has been well described by Moore's law, but further shrinkage of circuit size will halt by 2012 because of quantum mechanical effects. Quantum computing provides a way to circumvent this apparent roadblock and use these quantum effects to advantage. Atomic-scale devices, although promising, present major challenges in how to achieve spatial control and stability.

Dendrimers are highly branched three-dimensional nanoscale molecular objects of the same size and weight as traditional polymers. However, dendrimers are synthesized in a stepwise fashion, allowing for extremely precise control of their size and geometry. In addition, the chemical reactivity and properties of their periphery and core can be controlled easily and independently. Dendrimers are already being used in molecular recognition, nanosensing, light harvesting, and optoelectrochemical devices. Because they are built up layer by layer and the properties of any individual layer can be controlled through selection of the monomer, they are ideal building blocks in nanochemistry for the creation of more complex three-dimensional structures.39

Nanocrystals are crystals of nanometer dimensions, usually consisting of aggregates of a few hundred to tens of thousands of atoms combined into a cluster. Nanocrystals have typical dimensions of 1 to 50 nanometers (nm), and thus they are intermediate in size between molecules and bulk materials and exhibit properties that are also intermediate. For example, the small size of semiconductor quantum "dots" leads to a shifted light emission spectrum through quantum confinement effects—with the magnitude of the shift being determined by the size of the nanocrystal. Nanocrystals are of great interest because of their promise in high density data storage and in optoelectronic applications, as they can be efficient light emitters. Nanocrystals have also found applications as biochemical tags, as laser and optical components, for the preparation of display devices, and for chemical catalysis.

Recently, hollow carbon tubes of nanometer dimensions have been prepared and studied. These nanotubes constitute a new form of carbon, configurationally equivalent to a graphite sheet rolled into a hollow tube. Carbon nanotubes may be synthesized, with sizes ranging from a few microns to a few nanometers and with thicknesses of many carbon layers down to single-walled structures. The unique structure of these nanotubes gives them advantageous behavior relative to properties such as electrical and thermal conductivity, strength, stiffness, and toughness. Carbon nanotubes can also be functionalized with molecular recognition agents so that they may bind specifically to discrete molecular targets, allowing them to be used as high resolution AFM probes, as channels for materials separation, and as selective gates for molecular sensing.41

Like nanotubes, nanowires are very small rods of atoms, but nanowires are solid, dense structures, much like a conventional wire. Controlling the atom (material) used for building the wire, as well as its impurity doping, allows for control of its electrical conduction properties. Ultimately, chemists wish to fabricate and control nanowires that are a single atom or molecule in diameter, thus creating an unprecedented laboratory for studying how small structures affect electron transfer within the wire and between the wire and external agents. Clearly, nanowires offer the potential for creating very small IC components.

 

 5.3.6  Nanocomposites

Nanocomposites encompass a large variety of systems composed of dissimilar components that are mixed at the nanometer scale. These systems can be one-, two-, or three-dimensional; organic or inorganic; crystalline or amorphous. A critical issue in nanocomposite research centers on the ability to control their nanoscale structure via their synthesis. The behavior of nanocomposites is dependent on not only the properties of the components, but also morphology and interactions between the individual components, which can give rise to novel properties not exhibited by the parent materials. Most important, the size reduction from microcomposites to nanocomposites yields an increase in surface area that is important in applications such as mechanically reinforced components, nonlinear optics, batteries, sensors, and catalysts.

5.3.7 Lab on a Chip

Lab-on-a-chip devices are designed to carry out complex chemical processes at an ultrasmall scale, for example, synthesizing chemicals efficiently; carrying out biological, chemical, and clinical analyses; performing combinatorial chemistry; and conducting separations and analysis on a single, miniaturized device. When the amount of material in a sample is small or when it is highly toxic or dangerous, lab-on-a-chip devices offer an ideal way to complete complex chemical manipulations with extremely small sample sizes. Further, because the volumes used to carry solutions are extremely small, even very small sample amounts can be present in reasonable concentrations. Lab-on-a-chip technology has been aggressively pursued in biotechnology, where better ways to separate and analyze DNA and proteins are of great interest. It has also sparked great interest in the analysis of dangerous materials where it can be used, for example, by law enforcement or the military to analyze explosives and biological or chemical agents, while maintaining low risks.45

5.3.8 Nano-Electro-Mechanical Systems

Nano-electro-mechanical systems have also generated significant interest in the creation of tiny devices that can use electrochemical energy to carry out mechanical tasks, for example, nanomotors. One can envision that the coupling of chemical energy to mechanical transducers will enable the construction of devices that may be applied in medicine to treat illnesses, explore dangerous areas, or just reach places that larger-scale devices cannot. Research in this area focuses on understanding the preparation of nanoscale components to build such devices as well as the interactions between the components, especially the coupling between the electrochemical and mechanical components. In addition, a new understanding of effects such as friction and wear is required as the nanoscale components obey a different set of rules than their macroscopic counterparts.48

We are all aware of the use of ultrasound radiation in medicine, where it is being used mostly for diagnosis, and where more recently, focused ultrasound radiation is being used to destroy cancer cells. Less is known of its application in chemistry, despite the fact that it has applications across almost the entire breadth of that field. One of the main advantages in conducting sonochemical experiments is that it is very inexpensive to get started in the field. Let us first address the question of how 20 kHz radiation can rupture chemical bonds, and try to explain the role of a few parameters in determining the yield of asonochemical reaction and the unique products obtained when ultrasound radiation is used in materials science. To a spectroscopist using 1013–1014 Hz radiation to break chemical bonds, this is a puzzle. How is it that 20 kHz ultrasound radiation can do the same job? A number of theories were developed in order to explain how a 20 kHz sonic radiation could break chemical bonds. They all agree that the main event in sonochemistry is the creation, growth and collapse of a bubble that is formed in the liquid. The first puzzle is how such a bubble can be formed, considering the fact that the forces required to separate water molecules into a distance of two van-der Waals radii, would require a power of 105 W/cm2. On the other hand, it is well known that in a sonication bath, with a power of 0.3 W/cm2, water is already converted into hydrogen peroxide. Different explanations have been offered; they are all based on the existence of unseen particles, or gas bubbles, that decrease the intermolecular forces, enabling the creation of the bubble. These theories are supported by the experimental evidence, that when the solution undergoes ultrafiltration, before the application of the ultrasonic power, there is no sonochemistry. The second stage is the growth of the bubble, which occurs through the diffusion of solute vapour to the volume of the bubble. The third stage is the collapse of the bubble, that takes place when the bubble size reaches its maximum value. According to the hot-spot mechanism, this implosive collapse raises the local temperature to 5000 K and the pressures to a few hundred atmospheres.

These extreme conditions cause the rupture of chemical bonds. From the time we entered the field in 1993, we were intrigued by the fact that the products of many sonochemical reactions were in the form of amorphous nanoparticles. For example, K. Suslick, who was one of the initiators of the field, has demonstrated that the sonication of Fe(CO)5 as a neat liquid, or its solution in decalin, yielded 5–20 nanometer-sized amorphous iron particles. The reason for the amorphicity of the products is related to the high cooling rates (> 1011 K/s) obtained during the collapse of the bubble, which does not allow the products to organize and crystallize. These high cooling rates result from the fast collapse that takes place in less than a nanosecond. For this reason, a sonicated solution containing a volatile solute will always lead to amorphous products. However, the reason for the nanometer-sized particles is not yet clear.44

The estimated size of the collapsing bubble varies from ten to a few hundred microns. In addition to the region inside the bubble, where a gas phase reaction takes place upon its collapse, a second important region is of great significance. This is the interfacial region, which surrounds the collapsing bubble. Its width is calculated to be 200 nm, and the temperature reached after collapse is 1900 K7. Sonochemical reactions of nonvolatile compounds such as salts will occur in this region. In this case, the sonochemical reactions occur in the liquid phase. The products are either amorphous or crystalline nanoparticles, depending on the temperature in the ring region in which the reaction takes place.

We cannot mention here all the parameters (frequency, power, gas under which the sonication takes place, pressure of the gas, etc.) that affect the sonochemical yield and rate, and so we will address ourselves to that one important parameter, of temperature. The equation of an adiabatic implosion is Tmax = T0{Pex(g – 1)/Pbub}, where Tmax is the temperature reached after the collapse of the bubble, T0 is the temperature of the sonication bath, g = Cp/Cv, Pex is the external pressure equal to the m of the hydrostatic and acoustic pressure, and Pbub is the pressure of the gas inside the cavity, at the radius at which it collapses. The choice of a nonvolatile solvent (decalin, hexadecane, isodurene, etc.) guarantees that only the vapours of the solute can be found inside the cavitating bubble. Thus Pbub is practically the vapour pressure of the solute, and since it is found in the denominator, lower Pbub results in higher temperatures and faster reaction rates.

The conclusion is that the temperature affects the sonochemical reaction rate in two ways. On the one hand, lower temperatures cause a higher viscosity, which makes the formation of the bubble more difficult, and, on the other hand, the dominant effect is that at lower temperatures, higher rates will be achieved in sonochemical processes. This is why the sonic reaction involving volatile precursors is run at lower temperatures. Apparent negative activation energies are measured for sonochemical reactions.

Since 1996, when our first papers began to appear, we have published more than 120 papers describing the preparation of a large variety of nanomaterials, including metals, alloys, metal oxides, metal sulfides, metal nitrides, chalcagonides, metal–polymer composites, ceramic materials, dielectric materials, and others. In addition to the synthetic work, we have developed a number of fields that have emerged from our ability to prepare such a large variety of materials. Below we detail some of these research areas.

The self-assembly monolayer coating of many functional groups, especially thiols on surfaces and particles, have attracted many groups around the world. In about 60% of the reports, gold served as the substrate. 30% of the studies were conducted on silver and 10% on copper. We have coated Fe and Fe2O3 with long organic alkyl chains having a functional group at their ends. In addition to the regular questions, such as organization of the alkyl chains, chemical bonding versus physical absorption, and thermal stability, a new dimension, that of magnetic properties, has been added. The question asked was whether by changing the functional group, or the alkyl chain, it is possible to ‘tailor’ the magnetic properties. The answer obtained was a positive one. The magnetic properties depend strongly on the nature of the functional group bonded to the Fe or to the Fe2O3 nanoparticles. For example, the saturation magnetization (in fact, saturation is not observed and is only an approximate number) of thiols, carboxylic acids, and alcohols bonded to Fe is about 50– 80 emu/g Fe, whereas those having sulfonic and phosphonic acids are only 4–8 emu/g Fe. The comparison is made for chains with an equal number of carbon atoms. These differences are also reflected in the blocking temperatures of the coated particles.46

Another closely related field that we have developed is the sonochemical coating of submicron ceramic spheres (silica, alumina, and zirconia) by a large variety of nanonanoparticles. This was done by synthesizing the ceramic spheres by conventional techniques (like the Stobber method for silica particles). The spheres were then introduced into the sonication bath, mixed with the solution of the precursor, and ultrasonic radiation was passed through the solution for a predetermined time. In this way we were able to deposit on the surface of the ceramic spheres nanoparticles of metals (Ni and Co for example), metal oxides (Fe2O3, Mo2O5), rare earth oxides (Eu2O3, Tb2O3), semiconductors (CdS, ZnS), and Mo2C. A figure presenting a bare silica sphere and a coated silica sphere is shown in Figure5.1. The silica sphere is coated by nanocrystalline silver. Ultrasound irradiation of a slurry of silica submicrospheres, silver nitrate, and ammonia in an aqueous medium for 90 min, under an atmosphere of argon to hydrogen (95 : 5), yielded a silver–silica nanocomposite. By controlling the atmospheric and reaction conditions, we could achieve the deposition of metallic silver on the surface of the silica spheres.

Obtaining amorphous products is of importance in a number of fields of science, particularly in catalysis, where an amorphous nanoparticle is more active than the corresponding nanocrystalline particle having the same diameter. This was explained as being due to the dangling bonds active in the amorphous catalyst. Over the years, we have examined our products as catalysts in oxidation reactions, in the oxidation of cyclohexane. In a series of papers we compared the catalytic performance of sonochemically- made catalysts in this reaction.

In the last two years, we have concentrated our sonochemical efforts in two directions: sonochemical synthesis of mesoporous (MSP) materials, and the use of ultrasound radiation in the deposition of amorphous nanomaterials into the mesopores. We have developed a sonochemical method to prepare MSP silica, MSP titania, MSP YSZ (Yttria stabilized zirconia), and other MSP materials.

 

Figure 5.1. TEM image of silver nanoparticles deposited on silica spheres.

The sonochemical method is faster than the corresponding sol–gel preparation technique. The sonochemical products were shown to have thicker walls than those synthesized by the conventional methods. We have shown that the products are more hydrothermally stable than the sol–gel products. Our MSP titania has the highest surface area reported. In addition, we have used sonochemistry for the insertion of amorphous nanoparticles into the mesopores. We have deposited Mo2O5 into the mesopores of MCM-41 (MSP silica)10, and amorphous Fe2O3 (ref. 11) into the mesopores of MSP titania. Five physical methods were used to prove that the amorphous nanoparticles were indeed anchored onto the inner walls of the channels. The amount of Mo2O5 that was inserted in the mesopores was 45% by weight. An attempt to increase this amount showed that the excess is deposited outside the mesopores. In a recent paper submitted for publication, we reported the synthesis of MSP iron oxide. Its catalytic activity in the oxidation of cyclohexane is the highest obtained so far. It converts 36% of cyclohexane to cyclohexanol and cyclohexanone (5 : 1 ratio) at 70°C and under one atmosphere of oxygen.46

The second project is related to the sonochemical preparation of air-stable iron nanoparticles having a very high magnetization. Iron nanoparticles are pyrophoric and protecting them against oxidation is a challenge. In the process developed in our laboratory, we sonicated the solution of Fe(CO)5 in diphenylmethane. The as-prepared material is composed of iron nanoparticles coated by a polymer. Further annealing of the sample yields an airstable product. The characterization of the product and the stability studies are based on Mossbauer spectroscopy, XRD, and magnetization measurements.

Although some efforts in materials science are still directed towards developing new methods for the fabrication of nanomaterials, more attention is being directed presently to control of the size and shape of nanoparticles. We have demonstrated over the years that the control of the particle size is quite easy when using sonochemistry. It is accomplished simply by varying the concentration of the precursor in the irradiated solution. The more dilute the solution, the smaller the particles. The shapes of the products of the sonochemical process are less predictable. A major factor is the presence or absence of a surfactant. We can just mention that shapes such as olympic nanorings (BaFe12O19) ,anocylinders (GaOOH) , nanotubes (TiO2), nested inorganic fullerenes (Tl2O), and spheres have been among the shapes of the sonochemical products. The inorganic fullerenes in which the sonohydrolysis of group III A (Ga, Al, In and Tl) in the periodic table was conducted. An aqueous solution of the chlorides of these metals was sonicated at room temperature. The sonohydrolysis of TlCl3 yielded onion-like nested fullerenes. They were assigned to Tl2O which was one of the two sonication products. We have demonstrated that the high temperature developed during the sonication was the driving force for the sonohydrolysis. Heating an aqueous solution of GaCl3, for example, to 300°C, in a closed cell did not yield any product.

In addition to the catalytic applications mentioned above, the sonochemical products will be applied to two other fields. The first is their use as electrode building materials in rechargeable Li batteries, and the second is the fabrication of rare-earth doped optical fibers, which involves the sonochemical deposition of nanosized rareearth oxide on the surface of submicron silica spheres.48

In the cooperative project the scientists are working on a key problem of future transport technology: the release (“reforming”) of hydrogen from methanol in vehicles powered using fuel cells. The Fritz Haber Institute in Berlin heads the research project that involves a total of four Max Planck Institutes. The Zeit Trust finances the project. 

            The traditional combustion engine is going through a crisis: In future, it will not only have to stop generating so many fumes, it will also have to find a way to stop guzzling fuels on the basis of cheap petroleum. At the moment, fuel cells are seen as the most promising alternative to Otto and diesel engines. Using “cold” combustion, they are unmatched in the effectiveness with which they produce electric current to drive electric vehicle motors. Hydrogen is the most suitable substance to operate fuel cells and it is already used in traditional, slightly modified car motors, where it convincingly demonstrates its greatest advantage: No exhaust fuels, apart from harmless steam, are generated by the combustion, regardless of whether “hot” or “cold”. However, the “fuel of the future” is not yet suitable for general use – its main disadvantage is the storage: Whether in the form of gas or liquid, this is a technically complicated factor. Due to the low energy density in comparison to the volume, the thin gas must either be compressed by pressure or cooled to minus 253 °C before changing to a liquid state. Despite all this, the hydrogen tank still takes up a lot of space in the vehicle if one wishes to achieve ranges similar to those of petrol. At the moment, there are no signs of improvement in the direct storage of hydrogen. Furthermore, hydrogen is highly explosive and must avoid contact with air even in accidents. Nonetheless, an intermediate solution has appeared on the horizon: Using a chemical process described as “reforming” in vehicles, it is possible to release the hydrogen required to operate the fuel cells – using liquid fuels that have been used up until now. Methanol or petrol can both be used. Methanol reforming is less elaborate in a technical sense and can be realized in a fairly simple manner by reversing the chemical production process, the synthesis of methanol. But this is where the problem lies: traditional methods produce not only hydrogen, but also carbon monoxide. This is an extremely effective “poison” for fuel cells, effective even in tiny concentrations, which makes them unable to function. Among others, Max Planck scientists now intend to clarify the question whether there is a catalyst that can split methanol under technically suitable conditions without forming carbon monoxide, and whether nanocrystalline copper particles would be suitable for this.49

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