The new word "Nanomaterial" literally means that it is the material with at least one of its dimensions in the range of a nanometer.
The material need not be so small that it cannot be seen; it can be a large surface or a long wire whose thickness is in the scale of nanometers; the range could be 1 to 103 nanometers.
However this is only a trivial guess answer expected from a student of general education.
The Nanomaterials are of our interest not simply because their size is exactly or roughly one nanometer as suggested by the name. They are so named because the size, at and below which many materials exhibit interesting properties , falls on the scale of a few orders of a nanometer. We have to seek the speciality of this small size to get the most apppropriate difinition of the Nanomaterial: Suppose we cut any solid material of the some volume, say gold biscuit of 100 centimetre cube, into pieces: then each piece will have the same properties of the gold but for, ofcourse, the size. If they are cut further, the small pieces still possess the same qualities of the gold. Thus the macroscopic solid pieces of the materials possess size independent properties. The density, color, hardness, melting point, resistivity etc of the material do not depend on the size. Now imagine that we can cut the material indefinitely into pieces of any small size that we wish; these pieces may be so small that we cannot see them individually. Also assume that we can pack them together like a spoonful powder so that we can see them collectively. Then it has been found that below a particular size of each piece, the properties of the material depend on the size; the same gold pieces, each smaller than a particular size, will no longer be yellow; they may be red or violet collectively, depending on the particle size. This surprising fact is illustrated by the colour photos of samples.
Now-a-days to divide the matter into extremely small particles is not at all hypothetical; we have well established methods of preparing such small pieces and utilizing their special properties; in fact, this is the nanotechnology attracting top class industries throughout the globe to invest billions of dollars expecting roaring profits.
Now we may state the most appropriate definition:
A nanomaterial is that which is made up of particles each of size small enough to exhibit size dependent properties and large enough to form a solid state structure.
This matter will be in the gaseous state if its atoms or molecules are free from each other. In this field of nanotechnology, we are interested in solid state materials; hence nanoparticles are necessarilylarger than diatomic or triatomic molecules. They measure intermediate in size and posses transitional properties between isolated small molecules and bulk materials of the same composition. These new properties of the materials are size controlled and exploited to design the nanodevices, the smallest human made solid state structures of the present day technology. Biomaterials are naturally existing nanomaterials. Many universities, research institutes and prospective industries are preparing a variety of nanomaterials.
25.8.08
NANOFABRICATION
There are two basic approaches for nanoparticle manufacturing: The 'Top-down' and the 'Bottom-up' nanofabrications. Obviously, in the first approach, we start with a bulk material and go on grinding or etching it to nanoscale by some suitable techniques. In the second process, we start with the individual atoms or molecules and apply suitable techniques to build up the nanoparticles. There are many methods under each approach.
The nanoparticles are prepared in different forms: A nanoparticle with only one of its three dimensions in the nanoscale is called a quantum well, one with two dimensions in the nanorange is called a quantum wire or nanowire and one with all its three dimensions in the nanorange is known as a quantum dot. The word quantum, used in this context, is to indicate that the drastic changes in the material properties are due to quantum mechanical effects prevailing in the ultra small domains. There are also nanostructures in the form of nanoclusters and nanotubes used for different purposes. There are many methods of nanofabrication. While each method will have its own advantage, some may be more suitable than others for processing materials of particular composition.
1. Attrition Milling: Some companies have developed 'Stirred Ball Mills for grinding certain materials into fine powders. The machine consists of special steel balls in a water cooled steel vessel and a high speed motor driving shaft with arms.
The vessel is the stationary tank; the balls are stirred and act as the grinding media. The material to be powdered is kept in the media. When the machine operates, the media exerts both shearing and impact forces on the material and grinds it. This kind of milling is used to prepare silicon carbide nanoparticles. The grinding in the mill is carried with different fluids like ethylene glycol, n-hexane, xylene, butanol and water with surface active agent (surfactant). The size of the nanoparticles obtained in the process depends on many factors like type of SiC, grinding hour, the weight ratio of the steel balls to the initial SiC particles, fluid media etc. The nanoparticles produced by milling are contaminated by iron etc. The powder will be purified by treating with hydrochloric acid. The size of the SiC nanoparticles obtained is in the range of (30-550)nm. Nanoparticles of Alumina and Zirconia are also prepared by this process.
2. Sol-Gel process: Initially a homogeneous solution (Sol) of a material is prepared and used as a precusor for nanofabrication. The sol is deposited on a suitable substrate by spraying, dipping or spnning. It is kept for a while for geletion(Gel). During this 'gelation time' the material gradually looses its fluidity and undergoes a transition from viscous liquid state to elastic solid state. This time depends on the viscosity, temperature etc of the solution. The desired nanoparticles are finally fabricated from the gel by further processing of heat treatment etc. Some examples of the materials processed by this method are Tetraethyl Orthosilicate, Trimethyl Borate, Aluminium Sec-Butoxide, Titanium Iso-Propoxide, Zirconium Iso-Propoxide etc.
3. Nanoscale Lithography: The word lithography derived its meaning from sculpturing figures from big stones. The basic principle of the process is similar for both the nanostructures and the well known microchips but for selection of suitable agents. It makes use of radiation passing through a template and shining a surface of the material to be fabricated. The material is coated with a radiation-sensitive resist and is seated on a substrate. The desired pattern is impressed on the surface which is then masked. The rest of the material and the excess resist are etched out by chemical processing. Finally the mask is removed to obtain the nanostructure of the desired size and shape. In the case of nanofabrication, electron beam is used insted of light. A typical resist is the polymer polymethyl methacrylate. An example for the material fabricated by this process is GaAs. Other types of radiations like neutral atom beams are also employed in this process for some other materials(e.g.,Li, Na and k). This sequence of the steps in this process used for preparing a quantum dot or quantum wire from an ilnitial quantum well of the material is illustrated in the figure below:
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Nanoscale Lithography: (a) initial quantum well, coated with a resist, on a substrate; (b) radiation through a shielding template; (c) configuration after dissolving irradiated portion of resist by developer; (d) disposition after addition of etching mask; (e) mask after removal of remainder of resist; (f) configuration after etching away the excess quantum-well material; (g) final nanostructure on substrate after removal of etching mask.
There are some variations in the lithographic techniques. One of them is the so called Dip pen Nanolithography (DPN) developed by Chad Mirkin and his collaborators at Northwestern University. It uses Atomic Force Microscope (AFM) tip as the nano-pen. The atoms or molecules stored at the top in the AFM tip serve as reservoir of nano-ink. The material to be nanofabricated is placed on a substrate. Then the AFM tip is moved as a scanning probe on the surface of the material. The molecules transferred from the tip to the surface will affect the surface. This is like writing as we wish and is useful to construct any arbitrary nanostructures. The advantage of this technique is that any thing can be used as nano-ink to process any material. But it is a comparatively slow process.
4. Molecular Recognition: Some molecules have ability to attract and bind to some other specific molecules. This is known as molecular recognition. This process takes place in many bio-systems. For example, it is responsible for allergies caused in human beings. Chemistry of molecular recogniltion is a key factor in bottom - up nanofabrication. It is especially useful in the preparation of drugs,adhesives, paints etc.
5. Self- Assembly: In chemistry, especially in pharmaceutical chemistry, molecular synthesis is a routine job. Specific molecules are synthesized for different purposes by manipulating the necessary atoms one by one through chemical reactions. This process is very slow and cumbersome, more so when used at nanoscale. The self-assembly technique cleverly takes the advantage of one natural phenomena to synthesize the nanomolecules. The idea is that the molecules always seek the lowest energy available to them; if bonding between adjacent molecules accomplishes this, they will bond themselves. If necessary they will also reorient their physical positions for this purpose. Realizing this, self-assembly techniques are used for making nanostructured objects by selecting proper components which naturally organize themselves. In this process, particular atoms or molecules are introduced onto a surface or onto a preconstructed nanostructure. Then the molecules align themselves into a particular positions forming bonds in order to minimize their energy. Thus large quantities of nanostructures can be prepared.
The nanoparticles are prepared in different forms: A nanoparticle with only one of its three dimensions in the nanoscale is called a quantum well, one with two dimensions in the nanorange is called a quantum wire or nanowire and one with all its three dimensions in the nanorange is known as a quantum dot. The word quantum, used in this context, is to indicate that the drastic changes in the material properties are due to quantum mechanical effects prevailing in the ultra small domains. There are also nanostructures in the form of nanoclusters and nanotubes used for different purposes. There are many methods of nanofabrication. While each method will have its own advantage, some may be more suitable than others for processing materials of particular composition.
1. Attrition Milling: Some companies have developed 'Stirred Ball Mills for grinding certain materials into fine powders. The machine consists of special steel balls in a water cooled steel vessel and a high speed motor driving shaft with arms.
The vessel is the stationary tank; the balls are stirred and act as the grinding media. The material to be powdered is kept in the media. When the machine operates, the media exerts both shearing and impact forces on the material and grinds it. This kind of milling is used to prepare silicon carbide nanoparticles. The grinding in the mill is carried with different fluids like ethylene glycol, n-hexane, xylene, butanol and water with surface active agent (surfactant). The size of the nanoparticles obtained in the process depends on many factors like type of SiC, grinding hour, the weight ratio of the steel balls to the initial SiC particles, fluid media etc. The nanoparticles produced by milling are contaminated by iron etc. The powder will be purified by treating with hydrochloric acid. The size of the SiC nanoparticles obtained is in the range of (30-550)nm. Nanoparticles of Alumina and Zirconia are also prepared by this process.
2. Sol-Gel process: Initially a homogeneous solution (Sol) of a material is prepared and used as a precusor for nanofabrication. The sol is deposited on a suitable substrate by spraying, dipping or spnning. It is kept for a while for geletion(Gel). During this 'gelation time' the material gradually looses its fluidity and undergoes a transition from viscous liquid state to elastic solid state. This time depends on the viscosity, temperature etc of the solution. The desired nanoparticles are finally fabricated from the gel by further processing of heat treatment etc. Some examples of the materials processed by this method are Tetraethyl Orthosilicate, Trimethyl Borate, Aluminium Sec-Butoxide, Titanium Iso-Propoxide, Zirconium Iso-Propoxide etc.
3. Nanoscale Lithography: The word lithography derived its meaning from sculpturing figures from big stones. The basic principle of the process is similar for both the nanostructures and the well known microchips but for selection of suitable agents. It makes use of radiation passing through a template and shining a surface of the material to be fabricated. The material is coated with a radiation-sensitive resist and is seated on a substrate. The desired pattern is impressed on the surface which is then masked. The rest of the material and the excess resist are etched out by chemical processing. Finally the mask is removed to obtain the nanostructure of the desired size and shape. In the case of nanofabrication, electron beam is used insted of light. A typical resist is the polymer polymethyl methacrylate. An example for the material fabricated by this process is GaAs. Other types of radiations like neutral atom beams are also employed in this process for some other materials(e.g.,Li, Na and k). This sequence of the steps in this process used for preparing a quantum dot or quantum wire from an ilnitial quantum well of the material is illustrated in the figure below:
.bmp.jpg)
Nanoscale Lithography: (a) initial quantum well, coated with a resist, on a substrate; (b) radiation through a shielding template; (c) configuration after dissolving irradiated portion of resist by developer; (d) disposition after addition of etching mask; (e) mask after removal of remainder of resist; (f) configuration after etching away the excess quantum-well material; (g) final nanostructure on substrate after removal of etching mask.
There are some variations in the lithographic techniques. One of them is the so called Dip pen Nanolithography (DPN) developed by Chad Mirkin and his collaborators at Northwestern University. It uses Atomic Force Microscope (AFM) tip as the nano-pen. The atoms or molecules stored at the top in the AFM tip serve as reservoir of nano-ink. The material to be nanofabricated is placed on a substrate. Then the AFM tip is moved as a scanning probe on the surface of the material. The molecules transferred from the tip to the surface will affect the surface. This is like writing as we wish and is useful to construct any arbitrary nanostructures. The advantage of this technique is that any thing can be used as nano-ink to process any material. But it is a comparatively slow process.
4. Molecular Recognition: Some molecules have ability to attract and bind to some other specific molecules. This is known as molecular recognition. This process takes place in many bio-systems. For example, it is responsible for allergies caused in human beings. Chemistry of molecular recogniltion is a key factor in bottom - up nanofabrication. It is especially useful in the preparation of drugs,adhesives, paints etc.
5. Self- Assembly: In chemistry, especially in pharmaceutical chemistry, molecular synthesis is a routine job. Specific molecules are synthesized for different purposes by manipulating the necessary atoms one by one through chemical reactions. This process is very slow and cumbersome, more so when used at nanoscale. The self-assembly technique cleverly takes the advantage of one natural phenomena to synthesize the nanomolecules. The idea is that the molecules always seek the lowest energy available to them; if bonding between adjacent molecules accomplishes this, they will bond themselves. If necessary they will also reorient their physical positions for this purpose. Realizing this, self-assembly techniques are used for making nanostructured objects by selecting proper components which naturally organize themselves. In this process, particular atoms or molecules are introduced onto a surface or onto a preconstructed nanostructure. Then the molecules align themselves into a particular positions forming bonds in order to minimize their energy. Thus large quantities of nanostructures can be prepared.
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