Atomic and Molecular Basis of Nanotechnology

The molecular theory of matter starts with quantum mechanics and statistical mechanics. According to the quantum mechanical Heisenberg Uncertainty Principle the position and momentum of an object cannot simultaneously and precisely be determined [8]. Then the first question that may come into mind is, how could one be able to brush aside the Heisenberg Uncertainty Principle, Figure 2, to work at the atomic and molecular level, atom by atom as is the basis of nanotechnology.

The Heisenberg Uncertainty Principle helps determine the size of electron clouds, and hence the size of atoms. According to Werner Heisenberg "The more precisely the POSITION is determined, the less precisely the MOMENTUM is known". Heisenberg's Uncertainty

Principle applies only to the subatomic particles like electron, positron, photon, etc. It does not forbid the possibility of nanotechnology, which has to do with the position and momentum of such large particles like atoms and molecules. This is because the mass of the atoms and molecules are quite large and the quantum mechanical calculation by the Heisenberg Uncertainty Principle places no limit on how well atoms and molecules can be held in place [8].

Although we have long been aware of, and many investigators have been dealing with, "nano" sized entities; the historic birth of the nanotechnology is commonly credited to Feynman. Historically nanotechnology was for the first time formally recognized as a viable field of research with the landmark lecture delivered by Richard P. Feynman, the famous Noble Laureate physicist on December 29th 1959 at the annual meeting of the American Physical Society [9]. His lecture was entitled "There's Plenty of Room at the Bottom - An invitation to enter a new field of physics". Feynman stated in his lecture that the entire encyclopedia of Britannica could be put on the tip of a needle and, in principle, there is no law preventing such an undertaking. Feynman described then the advances made in this field in the past and he envisioned the future for nanotechnology. His lecture was published in the February 1960 issue of Engineering & Science quarterly magazine of California Institute of Technology.

Momentum Position Distribution Distribution

Figure 2. Heisenberg Uncertainty Principle.

In his talk Feynman also described how the laws of nature do not limit our ability to work at the molecular level, atom by atom. Instead, he said, it was our lack of the appropriate equipment and techniques for doing so. Feynman in his lecture talked about "How do we write small?", "Information on a small scale", possibility to have "Better electron microscopes" that could take the image of an atom, doing things small scale through "The marvelous biological system", "Miniaturizing the computer", "Miniaturization by evaporation" example of which is thin film formation by chemical vapor deposition, solving the "Problems of lubrication" through miniaturization of machinery and nanorobotics, "Rearranging the atoms" to build various nanostructures and nanodevices, and behavior of "Atoms in a small world" which included atomic scale fabrication as a bottom-up approach as opposed to the top-down approach that we are accustomed to [10]. Bottom-up approach is self-assembly of machines from basic chemical building blocks which is considered to be an ideal through which nanotechnology will ultimately be implemented. Top-down approach is assembly by manipulating components with much larger devices, which is more readily achievable using the current technology.

It is important to mention that almost all of the ideas presented in Feynman's lecture and even more, are now under intensive research by numerous nanotechnology investigators all around the world. For example, in his lecture Feynman challenged the scientific community and set a monetary reward to demonstrate experiments in support of miniaturizations. Feynman proposed radical ideas about miniaturizing printed matter, circuits, and machines. "There's no question that there is enough room on the head of a pin to put all of the Encyclopedia Britanica" he said. He emphasized "I'm not inventing antigravity, which is possible someday only if the laws (of nature) are not what we think" He added "I am telling what could be done if the laws are what we think; we are not doing it simply because we haven't yet gotten around to it." Feynman's challenge for miniaturization and his unerringly accurate forecast was met forty years later, in 1999, [11] by a team of scientists using a nanotechnology tool called Atomic Force Microscope (AFM) to perform Dip Pen Nanolithography (DPN) the result of which is shown in Figure 3. In this DPN an AFM tip is simply coated with molecular 'ink'

and then brought in contact with the surface to be patterned. Water condensing from the immediate environment forms a capillary between the AFM tip and the surface. Work is currently under way to investigate the potential of the DPN technique as more than a quirky tool for nanowriting, focusing on applications in microelectronics, pharmaceutical screening, and biomolecular sensor technology.

Feynman in 1983 talked about a scaleable manufacturing system, which could be made to manufacture a smaller scale replica of itself [12]. That, in turn would replicate itself in smaller scale, and so on down to molecular scale. Feynman was subscribing to the "Theory of Self-Reproducing Automata" proposed by von Neumann the 1940's eminent mathematician and physicist who was interested in the question of whether a machine can self-replicate, that is, produce copies of itself (see [13] for details). The study of man-made self-replicating systems has been taking place now for more than half a century. Much of this work is motivated by the desire to understand the fundamentals involved in self-replication and advance our knowledge of single-cell biological self-replications.

Some of the other recent important achievements about which Feynman mentioned in his 1959 lecture include the manipulation of single atoms on a silicon surface [14], positioning single atoms with a scanning tunneling microscope [15] and the trapping of single, 3 nm in diameter, colloidal particles from solution using electrostatic methods [16].

In early 60's there were other ongoing research on small systems but with a different emphasis. A good example is the publication of two books on "Thermodynamics of Small Systems" by T.L. Hill [17] in early 1960s. Thermodynamics of small systems is now called "nanothermodynamics" [18]. The author of this book was privileged to offer a short course on nanothermodynamics to a large group of university professors, other scientists and graduate students during last May [19].

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Figure 3. Demonstration of lithography miniaturization challenge by the scientists at the Northwestern University [11] as predicted by Feynman in 1959 using an AFM tip to write a paragraph of nanometer-sized letters with a single layer of mercaptohexadecanoic acid on a gold surface. Contrast is enhanced by surrounding each letter with a layer of a second "ink"--octadecanethiol.

In 1960s when Feynman recognized and recommended the importance of nanotechnology the devices necessary for nanotechnology were not invented yet. At that time, the world was intrigued with space exploration, discoveries and the desire and pledges for travel to the moon, partly due to political rivalries of the time and partly due to its bigger promise of new frontiers that man had also not captured yet. Research and developments in small (nano) systems did not sell very well at that time with the governmental research funding agencies and as a result the scientific community paid little attention to it.

It is only appropriate to name the nanometer scale "the Feynman (0nman) scale" after Feynman's great contribution and we suggest the notation "0" for it like Ä as used for Angstrom scale and ß as used for micron scale.

One Feynman (0 ) = 1 Nanometer (nm)= 10 Angstroms (A)= 10-3 Micron (ß) = 10-9 Meter (m)

Some Recent Key Inventions and Discoveries

Scanning Tunneling Microscope: Nanotechnology received its greatest momentum with the invention of scanning tunneling microscope (STM) in 1985 by Gerd K. Binnig and Heinrich Rohrer, staff scientists at the IBM's Zürich Research Laboratory [20]. That happened forty-one years after Feynman's predictions. To make headway into a realm of molecule-sized devices, it would be necessary to survey the landscape at that tiny scale. Binning and Rohrer's scanning tunneling microscope offered a new way to do just that.

STM allows imaging solid surfaces with atomic scale resolution. It operates based on tunneling current, which starts to flow when a sharp tip is mounted on a piezoelectric scanner approaches a conducting surface at a distance of about one nm (1 0). This scanning is recorded and displayed as an image of the surface topography. Actually the individual atoms of a surface can be resolved and displayed using STM.

Atomic Force Microscope: After the Nobel Prize award in 1986 to Binnig and Rohrer for the discovery of STM it was quickly followed by the development of a family of related techniques which, together with STM, may be classified in the general category of Scanning Probe Microscopy (SPM) techniques. Of the latter technologies, the most important is undoubtedly the atomic force microscope (AFM) developed in 1986 by Binnig, Quate and Gerber [21]. Figure 4 shows schematic of two typical AFMs.

An AFM, as shown in Figure 4, is a combination of the principle of STM and the stylus profilometer. It enables us to study non-conducting surfaces, because it scans van der Waals forces with its "atomic" tips. Presently several vendors are in the market with commercial AFMs.

AFM and STM possess three-dimensional resolutions up to the atomic scale which cannot be met by any other microscope. The AFMs sold by most manufacturers are generally user-friendly and they produce detailed images. AFM has found versatile applications in nanotechnology as well as other fields of science and engineering. The main components of this tool are thin cantilever with extremely sharp (1-10 nm [0] in radius) probing tip, a 3D piezo-electric scanner, and optical system to measure deflection of the cantilever. When the tip is brought into contact with the surface or in its proximity, or is tapping the surface, it being affected by a combination of the surface forces (attractive and repulsive). Those forces cause cantilever bending and torsion, which is continuously, measures via the deflection of the reflected laser beam.

3D scanner moves the sample or, in alternative designs, the cantilever, in 3 dimensions thus scanning predetermined area of the

Laser Diode

Quad

Photodiode

Scale

; Spring

Styli

Laser Diode

Lenz Light

; Spring

Tip T Cantilever

Lenz Light

Tip T Cantilever

Piezo

Electric

Scanner

Stylus Profilometer Atomic Force Concept Microscope

Piezo

Electric

Scanner

Figure 4. Schematic of a typical AFM and its function as compared with a stylus profilometer. As it is shown an AFM has similarities to a conventional stylus profilometer, but with a much higher resolution in nano scale. In the right hand side pictures of two AFMs are shown.

surface. A vertical resolution of this tool is extremely high, reaching 0.01 nm [0], which is on the order of atomic radius.

Diamondoids: The smallest diamondoid molecule was first discovered and isolated from a Czechoslovakian petroleum in 1933. The isolated substance was named adamantane, from the Greek for diamond. This name was chosen because it has the same structure as the diamond lattice, highly symmetrical and strain free as shown in Figure 5. It is generally accompanied by small amounts of alkylated adamantanes: 2-methyl-; 1-ethyl-; and probably 1-methyl-; 1,3-dimethyl; and others. From the bionanotechnology point of view diamondoids are in the category of organic nanostructures.

Adamentane Diamentane Trimentane

Adamentane Diamentane Trimentane

Tetram e nta n es

Figure 5. Chemical structures of diamondoid molecules. These compounds have diamond-like fused ring structures which can have many applications in nanotechnology. They have the same structure as the diamond lattice, i.e., highly symmetrical and strain free. The rigidity, strength and assortment of their 3-d shapes make them valuable molecular building blocks.

Tetram e nta n es

Figure 5. Chemical structures of diamondoid molecules. These compounds have diamond-like fused ring structures which can have many applications in nanotechnology. They have the same structure as the diamond lattice, i.e., highly symmetrical and strain free. The rigidity, strength and assortment of their 3-d shapes make them valuable molecular building blocks.

The unique structure of adamantane is reflected in its highly unusual physical and chemical properties. The carbon skeleton of adamantane comprises a small cage structure. Because of this, adamantane and diamondoids in general are commonly known as cage hydrocarbons. In a broader sense they may be described as saturated, polycyclic, cage-like hydrocarbons. The diamond-like term arises from the fact that their carbon atom structure can be superimposed upon a diamond lattice. The simplest of these polycyclic diamondoids is adamantane, followed by its homologues diamantane, tria-, tetra-, penta- and hexamantane.

Diamondoids have diamond-like fused ring structures which can have applications in nanotechnology. They have the same structure as the diamond lattice, i.e., highly symmetrical and strain free. Diamondoids offer the possibility of producing variety of nanostructural shapes. We expect them to have the potential to produce possibilities for application as molding and cavity formation characteristics due to their organic nature and their sublimation potential. They have quite high strength, toughness, and stiffness compared to other known molecule.

Diamondoids are recently named as the building blocks for nanotechnology [22]. Here is a partial list of applications of diamondoids in Nanotechnology and other fields [23]:

• Antiviral drug

• Cages for drug delivery

• Designing molecular capsules

• Drug Targeting

• Gene Delivery

• In designing an artificial red blood cell, called "Respirocyte"

• In host-guest chemistry and combinatorial chemistry

• In Nanorobots

• Molecular machines

• Molecular Probe

• Nanodevices

• Nanofabrication

• Nanomodule

• Organic molecular building blocks in formation of nanostructures

• Pharmacophore-based drug design

• Positional assembly

• Preparation of fluorescent molecular probes

• Rational design of multifunctional drug systems and drug carriers

• Self-assembly: DNA directed self-assembly

• Shape-targeted nanostructures

• Synthesis of supramolecules with manipulated architecture

• Semiconductors which show a negative electron affinity

Buckyballs: By far the most popular discovery in nanotechnology is the Buckminsterfullerene molecules. Buckminsterfullerene (or fullerene), C60, as is shown in Figure 6 is another allotrope of carbon (after graphite and diamond), which was discovered in 1985 by Kroto and collaborators [24]. These investigators used laser evaporation of graphite and they found Cn clusters (with n>20 and even-numbers) of which the most common were found to be C60 and C70. For this discovery by Curl, Kroto and Smalley were awarded the 1996 Nobel Prize in Chemistry. Later fullerenes with larger number of carbon atoms (C76, C80, C240, etc.) were also discovered.

Since the time of discovery of fullerenes over a decade and a half ago, a great deal of investigation has gone into these interesting and unique nanostructures. They have found tremendous applications in nanotechnology. In 1990 a more efficient and less expensive method to produce fullerenes was developed by Kratchmer and collaborators [25]. Further research on this subject to produce less expensive fullerene is in progress [26]. Availability of low cost fullerene will pave the way for

Naiuotube Diamond

Naiuotube Diamond

Figure 6. The four allotropes of carbon.

Figure 6. The four allotropes of carbon.

further research into practical applications of fullerene and its role in nanotechnology.

Carbon Nanotubes: Carbon nanotubes were discovered by Iijima in 1991 [27] using an electron microscope while studying cathodic material deposition through vaporizing carbon graphite in an electric arc-evaporation reactor under an inert atmosphere during the synthesis of Fullerenes [28]. The nanotubes produced by Iijima appeared to be made up of a perfect network of hexagonal graphite, Figure 6, rolled up to form a hollow tube.

The nanotube diameter range is from one to several nanometers which is much smaller than its length range which is from one to a few micrometers. A variety of manufacturing techniques has since been developed to synthesize and purify carbon nanotubes with tailored characteristics and functionalities. Controlled production of singlewalled carbon nanotubes is one of the favorite forms of carbon nanotube which has many present and future applications in nanoscience and nanotechnology. Laser ablation chemical vapor deposition joined with metal-catalyzed disproportionation of suitable carbonaceous feedstock are often used to produce carbon nanotubes [29-31]. Figure 7 is the scanning electron microscope (SEM) images of a cluster of nanotubes recently produced through plasma enhanced chemical vapor deposition at two different temperatures [29].

Carbon nanotubes and fullerenes are shown to exhibit unusual photochemical, electronic, thermal and mechanical properties [32-35]. It is also shown that single-walled carbon nanotubes (SWCNTs) could behave metallic, semi-metallic, or semi-conductive one-dimensional objects [32], and their longitudinal thermal conductivity could exceed the in-plane thermal conductivity of graphite [33]. Very high tensile strength (~100 times that of steel) of ropes made of SWCNTs has recently been determined experimentally [34]. When dispersed in another medium, it is demonstrated that SWCNTs could retain their intrinsic mechanical attributes or even augment the structural properties of their medium host [35]. SWCNTs have similar electrical conductivity as copper and similar thermal conductivity as diamond.

There is a great deal of interest and activity in the present day to find applications for fullerene and carbon nanotube. There are many ongoing research activities to understand the characteristics of carbon nanotubes including their physicochemical properties, their stability and

Figure 7. Carbon nanotubes produced using plasma-enhanced chemical vapor deposition at various temperatures [29]. SEM images of deposited carbon nanotubes at (a) 650 oC, (b) 700 oC.

behavior under stress and strain, their interactions with other molecules and nanostructures and their utility for novel applications [36-40].

Cyclodextrins, Liposome and Monoclonal Antibody: At the same time that chemists, materials scientists and physicists have been experimenting with structures like carbon nanotubes and buckyballs and diamondoids, biologists have been making their own advances with other nanoscale structures like cyclodextrins [41], liposomes [42] and monoclonal antibodies [43]. These biological nanostructures have many applications including drug delivery and drug targeting.

Cyclodextrins, as shown in Figure 8, are cyclic oligosaccharides. Their shape is like a truncated cone and they have a relatively hydrophobic interior. They have the ability to form inclusion complexes with a wide range of substrates in aqueous solutions. This property has led to their application for encapsulation of drugs in drug delivery.

Figure 8. Chemical formula and structure of Cyclodextrins - For n=6 it is called a-CDx, n=7 is called P-CDx, n=8 is called y-CDx. Cyclodextrins are cyclic oligosaccharides. Their shape is like a truncated cone and they have a relatively hydrophobic interiors. They have the ability to form inclusion complexes with a wide range of substrates in aqueous solution. This property has led to their application for encapsulation of drugs in drug delivery.

Figure 8. Chemical formula and structure of Cyclodextrins - For n=6 it is called a-CDx, n=7 is called P-CDx, n=8 is called y-CDx. Cyclodextrins are cyclic oligosaccharides. Their shape is like a truncated cone and they have a relatively hydrophobic interiors. They have the ability to form inclusion complexes with a wide range of substrates in aqueous solution. This property has led to their application for encapsulation of drugs in drug delivery.

Liposome is a spherical synthetic lipid bilayer vesicle, created in the laboratory by dispersion of a phospholipid in aqueous salt solutions. Liposome is quite similar to a micelle with an internal aqueous compartment. Liposomes, which are in nanoscale size range, as shown in Figure 9, self-assemble based on hydrophilic and hydrophobic properties and they encapsulate materials inside. Liposome vesicles can be used as carriers for a great variety of particles, such as small drug molecules, proteins, nucleotides and even plasmids to tissues and into cells. For example, a recent commercially available anticancer drug is a liposome, loaded with doxorubicin, and is approximately 100-nanometer in diameter.

NON POLAR TAIL POLAR HEAD

Figure 9. Cross section of a liposome - a synthetic lipid bilayer vesicle that fuses with the outer cell membrane and is used to transport small molecules to tissues and into cells.

NON POLAR TAIL POLAR HEAD

Figure 9. Cross section of a liposome - a synthetic lipid bilayer vesicle that fuses with the outer cell membrane and is used to transport small molecules to tissues and into cells.

A monoclonal antibody protein molecule consists of four protein chains, two heavys and two lights, which are folded to form a Y-shaped structure (see Figure 10). It is about ten nanometers in diameter. This small size is important, for example, to ensure that intravenously administered these particles can penetrate small capillaries and reach cells in tissues where they are needed for treatment. Nanostructures smaller than 20 nm can transit out of blood vessels.

Antibody & its Structure

Figure 10. An antibody is a protein (also called an immunoglobulin) that is manufactured by lymphocytes (a type of white blood cell) to neutralize an antigen or foreign protein.

Antibody & its Structure

Figure 10. An antibody is a protein (also called an immunoglobulin) that is manufactured by lymphocytes (a type of white blood cell) to neutralize an antigen or foreign protein.

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