Molecular Building Blocks MBBs in Nanotechnology

The most fundamentally important aspect of the bottom-up approach is that the nanoscale building blocks, because of their sizes of a few nms, impart to the nanostructures created from them new and possibly preferred properties and characteristics heretofore unavailable in conventional materials and devices. The band gap of nanometer-scale semiconductor structures increases as the size of the microstructure decreases, raising expectations for many possible optical and photonic applications. Considering that nanoparticles have much higher specific surface areas, thus in their assembled forms there are large areas of interfaces. One needs to know in detail not only the structures of these interfaces, but also their local chemistries and the effects of segregation and interaction among molecular building blocks (MBBs) and also between MBBs and their surroundings. Knowledge on means to control nanostructure sizes, size distributions, compositions, and assemblies are important aspects of nanoscience and nanotechnology.

The building blocks of all materials in any phase are atoms and molecules. All objects are comprised of atoms and molecules, and their arrangement and how they interact with one another define the properties of an object. Nanotechnology is the engineered manipulation of atoms and molecules in a user-defined and re-peatable manner to build objects with certain desired properties. To achieve this goal a number of molecules are identified as the appropriate building blocks of nanotechnology. These nanosize building blocks are intermediate-size systems lying between atoms and small molecules and microscopic and macroscopic systems. These building blocks contain a limited and countable number of atoms. They can be synthesized and designed atom by atom. They constitute the means of our entry into new realms of nanoscience and nanotechnology.

The controlled and directed organization of molecular building blocks and their subsequent assembly into nanostructures is one fundamental theme of bottom-up nanotechnology. Such an organization can be in the form of association, aggregation, arrangement, or synthesis of MBBs through van der Waals forces, hydrogen bonding, attractive intermolecular polar interactions, electrostatic interactions, hydrophobic effects, and so on. The ultimate goal of assemblies of nanoscale molecular building blocks is to create nanostructures with improved properties and functionality heretofore unavailable with conventional materials and devices. Fabrication of nanostructures demands appropriate methods and molecular building blocks.

The applications of MBBs would enable the practitioner of nanotechnology to design and build systems on a nanometer scale. These promising nanotechnology concepts have far-reaching implications (from mechanical to chemical processes, from electronic components to ultrasensitive sensors, from medical applications to energy systems, and from pharmaceutical to agricultural and food chain) and will have an impact on every aspect of our future.

Through the controlled and directed assembly of nanoscale molecular building blocks one should be able to alter and engineer materials with desired properties. For example, ceramics and metals produced through controlled consolidation of their MBBs have been shown to possess properties substantially improved and different from materials with coarse microstructures. Such different and improved properties include greater hardness and higher yield strength in the case of metals and better ductility in the case of ceramic materials [4,5].

It should be pointed out that MBBs with three linking groups, such as graphite, could only produce planar or tubular structures. MBBs with four linking groups can form three-dimensional diamond lattices. MBBs with five linking groups can create three-dimensional solids and hexagonal planes. The ultimate present possibility is MBBs with six linking groups. Adamantane (see Figure 3.1) and buck-yball (C60), are of the latter category which can construct cubic structures [5,6]. Such MBBs can have many applications in nanotechnology and they are of major interest in designing shape-targeted nanostructures, nanodevices, molecular machines [4-7], nanorobots, and synthesis of supramolecules with manipulated architectures. A far-reaching example of the possibility of using diamondoids is in the conceptual and exploratory design of an artificial red blood cell, called

Figure 3.1. Six linking groups of adamantane.

Respirocyte, which would have the ability to transfer respiratory gases and glucose [8-10].

In general nanotechnology, molecular building blocks are distinguished for their unique properties. They include, for example, graphite, fullerene molecules made of a number of carbon atoms (C60, C70, C76, etc.), carbon nanotubes, nanowires, nanocrystals, amino acids, and diamondoids [11]. All these molecular building blocks are candidates for various applications in nanotechnology.

The diamondoid family of compounds is one of the best candidates for molecular building blocks to construct organic nanostructures compared to other MBBs known so far [4-7]. Diamondoids offer the possibility of producing a variety of nanostructural shapes. They have quite high strength, toughness, and stiffness compared to other known MBBs. These are tetrahedrally symmetric stiff hydrocarbons that provide an excellent building block for positional (or robotic) assembly as well as for self-assembly. In fact, over 20,000 variants of diamondoids have been identified and synthesized and even more are possible [4-6], providing a rich and well-studied set of MBBs. Diamondoids have recently been named as the building blocks for nanotechnology [12].

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