NANOTECHNOLOGY BACKGROUND

Transcription

1 NANOTECHNOLOGY BACKGROUND Introduction and Overview Nanotechnology is defined as the study of the fundamental laws and theories of atoms and structures that have critical dimensions between 1 and 100 nanometers. But why is nanotechnology so unique? Chemists study elements and molecules with dimensions much smaller than nanometers, and chemistry has been around for hundreds of years. Yet here is the crux of the importance and splendor of nanotechnology. Chemists study particles and atoms and molecules that are found throughout nature, yet nanoscientists are able to use the molecules and elements from chemistry and engineer them to form different useful, incredibly small tools. The nanometer is at the conflux of human synthetic ability and natural occurring molecules. Anything that is smaller than a nanometer is just a vagrant molecule or random speck, so structures on the nanoscale are the smallest structures that humans can possibly make. So while it is possible to observe and define materials smaller than a nanometer, it is impossible to fabricate materials any smaller. As we delve deeper and deeper into the mysterious realms of nanotechnology, we find that things are not the same very small as they are on a normal scale. For example, this is a gear constructed on the nanoscale. It is about 425 micrometers small. Not quite on the nanoscale, but it still works. This gear works similarly to a normal gear you see, say perhaps on a bike. It can rotate, spin, drive, or give mechanical advantage. However, the material properties that you worry with a normal gear are quite different then the properties that are prevalent in a micro-scale gear. With macro-size gears forces such as friction, hardness, and heat transfer are all important in considering the design and engineering of the gear. When structures are on the nanoscale, there is so little surface contact between materials that friction and heat transfer are almost non-existent. However, forces that have little effect on the macrogear have a tremendous impact on how the nano-gear works. However, properties such as molecular polarity and wave-particle duality are examples of a few of these. Also, factors such as wind and water contact have little impact on bicycles gears, but can be disastrous to a nanosized gear.

2 The Science behind Nanotechnology Nanotechnology draws from disciplines such as chemistry, physics, mechanics, biology, and material science. It is important to have a basic idea of the concepts behind each of these disciplines. The Atomic Level All describable matter in the universe is comprised of three basic particles: the proton, neutron, and electron. While there are sub-atomic particles (quarks, leptons and the like) protons, neutrons, and electrons represent the simplest particles required to describe matter. Electrons are extremely light particles--electron mass is often disregarded entirely that have a negative charge. Protons are positively charged particles found in the nucleus, or center, of an atom. Neutrons are similar to protons in size but have no charge. Protons and neutrons make up the nucleus of the atom, while electrons orbit the nucleus. When two particles are close to each other, they interact by one of the most fundamental laws of nature know as Coulomb s law. By definition, two charged particles separated by distance r, the force acting between them is given as F=Q 1 Q 2 /r In this case, F is the force that is acting between the particles separated by distance r. The charges on the two particles are Q 1, Q 2. Note that if the two particles are both electrons, then the force will be positive. Positive forces imply that the two particles push away from each other, much like two north ends of a magnet. Likewise, a positively charged proton and a negatively charged electron have a negative force and therefore are attracted to each other. This explains how an atom is held together. Note also that the farther away two particles are (r gets bigger), the smaller the force acting on them. Coulomb s Law helps us explain why particles do what they do, and how atoms are held together. There are a total of 91 naturally occurring elements in our world, and they differ in the number of protons, neutrons, and electrons. Together in different combinations, they make up all natural matter in our world. The size of natural atoms are essentially the same, with the smallest being.1 nanometer, and the biggest being.22 nanometers. In an uncharged atom, the number of protons equals the number of electrons, thereby making the entire atom s charge neutral. However, often times protons and electrons differ in number, thereby giving the atom a positive or negative charge. For example, if an atom has 3 protons and 4 electrons, it has a net charge of - 1. These charged atoms are called ions, and they play an extremely important role in science. How can 91 different atoms make up the myriad of forms of matter we see every day in our world? The answer is in the structure and combination of the atoms. When two different atoms combine in a fixed structure, the result is called a molecule. There are millions of different atom combinations, and therefore millions of different molecules. Atoms bond to form

3 molecules by a variety of chemical ways. Chemical bonds such as covalent and ionic bonds are a key aspect of nanotechnology. Nanotechnologists can manipulate different molecules and bonds to form a certain desired function or structure. Because this is such a small scale, a chemical bond or reaction just might be the rotating gear or the moving lever that allow nanostructures to function properly. It is these structures that allow for different fabrication techniques, and the most interesting is self-assembly which allows molecule structures to form on their own. The Macroscopic Level Most of the 91 naturally occurring elements like to cluster with their own kind, and this process makes macro structures that contain billions and billions of structures of the same atom. Many of these large structures (relative to the nano scale that is) become hard, shiny, ductile structures called metals. We are all familiar with metals such as aluminum, titanium, gold, copper, silver, and different alloys. The unique aspects of metals are their ability to conduct electricity. This happens because the metals electrons leave their individual atoms and flow freely throughout the metal structure as a whole. Power lines, extension cords, and speaker wires are all examples of this concept. The huge groups of atoms that are not metals are usually lighter such as: graphite, coal, diamonds, and yellow sulfur. These materials are mostly insulators because they do not allow the free flow of electrons. Another two types of materials worth mentioning are polymers and ceramics. The most common polymers are plastics, which we obviously deal with everyday in many different structures and functions. Polymers are extremely long chains of carbon bonded to itself. They can be classified into two categories: crosslinked polymers and amorphous polymers. Crosslinked polymers are polymers that tend to connect to one another in parallel and represent the more rigid structures such as Polyvinyl Chloride (PVC piping); whereas amorphous polymers are polymers that wrap like spaghetti, forming pliable and rubbery materials such as a styrofoam cup. The naturally occurring polymers are substances such as DNA molecules, proteins, and polysaccharides. Polymers generally do not conduct electricity and are commonly used as insulators. Ceramics on the other hand are materials containing oxides, which are materials where there is one extending atom that is oxygen. Examples of ceramics include: clay, sand, firebrick, and a more commonly a toilet. Ceramics are also poor at conducting and are therefore used as insulators (although some ceramics can become superconductors if super-cooled). Biosystems Our bodies are interesting in the fact that they need many different trace elements such as zinc, copper, iodine, manganese, and selenium. However, around 95 percent of our bodies contain the four elements hydrogen, carbon, nitrogen, and oxygen. These four elements can form many varieties of bond types; therefore nature can use them to make many different nanostructures that carry out the complex processes of life. The polymers made by these natural processes are usually more complex, irregular polymers. There are four large categories of biological molecules: nucleic acids, proteins, carbohydrates, and a catchall category which are specialized molecules. Of course proteins and protein variations carry out most of the functions of our life processes; therefore they act as bio-machines. Nucleic acids come in two main types:

4 DNA and RNA. DNA is a well known double-helix structure that is probably the most important part of our bodies because it contains the code that controls all of the functions that are cells carry out every second. DNA contains four different base pairs: Adenine, Guanine, Cytosine, and Thymine (abbreviated A, G, C, and T respectively). These bases always form pairs with A and G always pairing together and C and T always pairing together (this property allows for many applications at the nano level as discussed later). The sequence of these pairs makes a readable code that is transported by RNA to the ribosomes within a cell. The ribosome will then make a certain protein that will carry out a certain function to continue the process of life. The next big class of macromolecules in biology is polysaccharides, which are long chains which form sugars. These sugars give the cells the energy they need to perform certain functions, however they have not been found to have a major use in nanotechnology. The fourth large classification is a sort of catchall category. This includes molecules we are all familiar with such as: water, carbon dioxide, oxygen, and nitric oxide. These small molecules perform many functions vital to survival and can have many applications in medical nanotechnology. Molecular Recognition This is where all of the above information comes together, well mostly. Molecules have certain shapes and charges, and this means that the molecules will be made of different numbers of electrons and have different electron masses. Also, Coulomb s law describes the attraction of oppositely charged particles, and this means that molecules have the ability to interact with one another through Coulombic forces. Many of the functions of our bodies are based on molecular recognition such as: allergies, olfactory sense, and pheromones. Molecular recognition is very important as we will learn later because of the ability to build nanostructures from the bottomup. That is using the properties of molecular recognition to allow the molecules to arrange themselves or at least only provide a certain structure or mold that they can follow. Here is a macroscale example, [Insert Analogy] Electrical Conduction This is one of the most interesting fundamentals of nanotechnology. Biologically, all of the processes that take place in our bodies are a result of nerve impulses. These impulses are caused by electrical impulses traveling down a nerve axon and in most cases causing a

5 neurotransmitter to release and a certain function to be performed. These electrical impulses are extremely crucial not only to our bodies, but in our everyday lives. Electrical conductance allows power to be delivered to our houses, cell phones, and any other object that requires energy to work on. As mentioned above, metals are extremely great conductors, most commonly copper. The defining equation for most cases of electricity evaluation is Ohm s Law: V = I*R A common analogy used for this concept is a river. I represents the flow of electrons, or current, and correlates with the flow of water down the river. R represents the resistance to the flow of electrons, or resistance, and correlates with rocks or maybe a dam that resists the flow of water down a river. Then of course V represents the force that moves the current of electrons, or voltage, and correlates with gravity or a pump that pushes the water down a river. This law is obeyed in most common circuits, however there are circuits that do not obey this law. These circuits fall into categories such as superconductors and semiconductors. These are circuit elements in which there is effectively no resistance and allow for maximum power usage and transfer. There are also certain nanostructures such as carbon nanotubes that are found to at least be semiconductors, which lead to exciting and interesting applications for these structures. Quantum Theory Newtonian physics guides the motion and properties of every object that we can see and experience at the macro scale. But there are other laws for governing motion at the nanoscale and below. Atoms and electrons do not act like we normally predict. Electrons have a unique property because they act both like a wave and a particle (wave-particle duality). So the ideas of classical mechanics have been replaced by a newer theory called quantum mechanics. We will not go into the extent of this theory, but we will discuss the main ideas and the important parts as they apply to nanotechnology. First off, at this scale energy cannot be added in a continuous way, it must be added in small chunks referred to as quanta (plural of quantum). For example, to change the charge on an ion, it can only be done one electron unit charge at a time. This theory governs nanoscale properties such as: how small a wire can be and still carry electrical charge, or how much energy we have to put into a molecule before it can change its charge state or act as a memory element. Optics This section describes how light interacts with matter. For example, the large molecule called phthalocyanine, which provides the blue color in jeans, can be changed to give greenish or purplish colors by modifying the chemical or geometric structure of the molecule. Matter can also transfer heat like a black car on a sunny day, or give off light energy like a light bulb or fireworks. Another property worth mentioning is that the smaller the metallic object, the quanta of energy that apply to them become larger.

6 Nanoscience Measuring Tools In order to work at the nano-scale, scientists and engineers must be able to observe and measure what they are working with. There are a variety of different tools which are used to accomplish these goals on the nanoscale. Scanning Probe Instruments: One of the most common forms of measuring nanostructures is using scanning probe instruments. In all scanning probe instruments, a nanoscale sized probe, or tip, slides across the surface to be measured. As the tip slides, different materials exert different forces on the tip, and computer software is able to tell where one material starts and the other stops. Think of running your finger across a metal plate and then across a rough piece of wood, it is quite easy to tell the difference. As the probe moves across the surface, it can measure a number of different forces. Atomic force microscopy (AFM) measures the actual physical force of the tip on the surface, much like the finger on the wood. Scanning tunneling microscopy (STM) measures the amount of electrical current between the tip and surface. Magnetic force microscopy (MFM) employs a magnetic tip that reads the magnetic structure of the surface. Spectroscopy: One of the oldest and most general techniques is a concept called spectroscopy. As the name infers, spectroscopy employs light in its measurements. We all have seen examples of a type of spectroscopy in the macro-world. X-ray machines pass extremely high energy light waves through an object and read the resulting scattering of the waves due to different materials. By using different energies of light, nanoscientists can analyze nanostructures. However, spectroscopy does have its drawbacks. Because visible light has a constant wavelength, it is impossible to measure objects that are smaller than that wavelength. Since the visible light wavelength ranges from about 400 nanometers to 900 nanometers, it is difficult to measure objects that are only a few nanometers in size. Because of these limitations, spectroscopy is generally employed in measuring nanostructures as a group. Electrochemistry: The concept of electrochemistry refers to how the application of electric current can change different chemical reactions and processes, and how chemical reactions can generate electric currents. This technique is common in nanofabrication, but can also be useful in nanostructure analysis. Instruments can analyze the different electrochemical properties of different materials in order to measure or observe them.