Introduction

Nanotechnology defined

Nanotechnology is the study of matter with dimensions between approximately 1 and 100 nanometers.   Unlike macroscopic or microscopic material visible to the human eye or microscopes, nanomaterials and structures exhibit novel properties.   The word nano comes from a Greek word meaning dwarf.   Technically speaking, however, nano denotes a factor of 10-9.   In SI units, 1 nanometer is a billionth of a meter.   To get some perspective on the nanoworld, take a look at this scale.


In essence, nanotechnology involves design and manipulation at the atomic scale to either perform functions that micro or macroscale materials cannot or to increase the efficiency of existing materials.   The research and development of current nanotechnological applications spans several disciplines:  physics, chemistry, engineering, biology, and computer science.  

Technological advances that make nanotechnology reality

Advances in the field of microscopy have enabled scientists and engineers to physically witness some of the topographical features of nanomaterials.   The atomic force microscope (AFM) can have a resolution as high as a fraction of a nanometer and thus is the foremost tool for imaging and manipulating at the nano level.   Even though the instrument is still called a microscope, the AFM does not allow the user to physically see atoms as if looking through a lens to see red blood cells rushing through veins.   The AFM scans over a sample with an ultra fine tip.   The sample is mounted to a scanner which moves in the X-Y plane.   The scanner also contains freedom to move in the z-direction in order to maintain a constant force between the tip and the sample.   As the tip moves over the sample, a laser-photodiode combination detects any displacement due to the tip's attraction or repulsion from the surface.   The displacement data as well as the X-Y data at which the displacement occured are stored in a computer.   The 3-D data can be recalled, plotted, and digitally enhanced on a computer.

The basics for grasping nano

Because nanotechnology involves manipulating the smallest building block of matter, the atom, it is important to grasp some basic aspects of physics and chemistry.   Chemistry deals with the composition of matter as well as what combinations of atoms yield the desired substances and behaviors.   Physics, however, is more concerned with the forces present in the atom itself as well interactive forces between other atoms and molecules.   Both physics and chemistry are needed to understand what happens at the atomic scale.   The atom is composed of three main particles: electrons, protons, and neutrons.   The nucleus of the atom contains the positively charged protons and uncharged neutrons, which also contribute to nearly all of the entire atoms weight.   Bohr's model of the atom portrays the electrons in circular orbits of various size orbiting around the nucleus.   The model that Bohr theorized is only a decent approximation of the hydrogen atom.   The blue particles in the nucleus are the protons, and the red particles are the neutrons.   As a result of Shcrödinger's equation, however, electrons can be seen as occupying a particular region while orbiting around the nucleus instead of having a fixed planet-like orbit.   Renowned physicist, Richard Feynman, described the electron regions as electron clouds.   A revised model shows the difference between Bohr's model and the electron clouds.   The orange sphere represents the s-orbital while the six blue lobes represent the three p-orbitals.   The sphere and the lobes represent the electron clouds in which the electrons can most frequently be found.   In other words, the electrons do not have a particular fixed orbit.   More complex atoms contain different orbital shapes as well as larger orbital size to accomodate the growing number of electrons.  

Bohr's model is not entirely inadequate.   Prior to Bohr, Planck found that an atom can absorb or emit energy (in the form of electromagnetic radiation) only in quantized amounts.   Furthermore, the frequency of the electromagnetic radiation either absorbed or emitted is proportional to the transmitted energy through Planck's constant, h = 6.626 × 10-34 J-s (E = hf, where f is frequency).   Planck used his findings to describe the emission of various wavelengths of light from hot or black-body objects.   Bohr used Planck's ealry quantum theory in the development of his atomic model.   According to Bohr, when an atom absorbs energy an electron can be excited from its stable state to an unstable state of higher energy.   In its transition to a higher energy state, the electron goes from a lower orbital to a higher orbital.   Energy must be emitted from the atom for the electron to achieve its initial state of stability.   The electronmagnetic radiation emitted from the atom can only have a specific amount of energy related to the difference in energy between the orbitals.   Thus, by Planck's equation, if there is constant amount of energy released from an excited atom, the corresponding eletromagnetic radiation can have one and only one frequency.   If the electromagnetic radiation happens to be in the visible spectrum, then the light will be a certain color. &nbs; The electromagnetic radiation does not have to be visible.   Depending on the atom, x-rays, ultraviolet, and infrared radiation can be emitted from an excited atom.  

Planck's research was developed further in Einstein's description of the photoelectric effect phenomenon.   Bohr's model serves as a good visualization of the production of photons for the photoelectric effect.   Einstein's photons are a more accurate description of Planck's quanta.   The photon exhibits properties of waves as well as particles; therefore, the photon can be thought of as an energy packet of electromagnetic radiation.   Not only can the photon be refracted and interfered as a wave, but also the photon can deliver a certain amount of energy just as a particle delivers energy in a collision.   The amount of deliverable energy, as a result of Planck's work, is only dependent on the frequency of the photon.   

Quantum theory is necessary for describing the properties of an individual atom as well as interatomic behaviour such as affinity for chemical bonding.   Four sets of quantum numbers are used to describe the current state of an atom.  

•   Principal quantum number ( n = 1, 2, 3 … ) — A larger value of n indicates a larger average distance from the nucleus. In Bohr’s atomic model, the principal quantum number can be thought of as shells with size proportional to n. The energy of an electron with a particular principal quantum number has been observed to have discrete values.



•   Orbital quantum number ( 0 ≤ l ≤ n − 1 ) — l gives the orbital angular momentum of an electron orbiting the nucleus.  The various resulting values of the angular momentum indicate different orbital shapes.  Therefore, l is used to describe the shape of an electron’s orbit within the shell.  If l = 0, the electron occupies the s-orbital.  l = 1 corresponds to the p-orbital, l = 2 to the d-orbital, and l = 3, the f-orbital.  As l increases, the orbit is more complex.



•   Magnetic quantum number ( −l ≤ ml ≤ l ) — the magnetic quantum number is used to give the possible energy states of an electron which occupies a certain orbital, l, of a particular shell, n. For example, if l = 1 (the p-orbital), ml can be -1, 0, or 1, indicating that electrons in the p-orbital have three available energy states.



•   Spin quantum number ( ms = -½, ½ ) — ms is the defining value of an electron’s unique quantum state. Within each possible energy state of an atom’s orbital, only two electrons can co-exist. If two electrons occupy an energy state, the spin or angular momentum of one electron is opposite of the other.


Example:  In the Bohr model animation, there are 8 total electrons. The first two electrons ( ms = -½, ½ ) occupy the only energy state ( ml = 0 ) available in s-orbital ( l = 0 ) of the first energy level ( n = 1 ). The other six electrons pair up to occupy the three energy states ( ml = −1, 0, 1 ) available in the p-orbital ( l = 1 ) of the first energy level.

The electrostatic forces present in the atom are caused by the interactions of the charged particles (i.e. the protons and electrons).   The protons and neutrons are held together in the nucleus by a very short yet strong nuclear force.   The electrons are attracted close to the nucleus by the positively charged protons but are kept from bombarding the nucleus by the repulsive forces present between the electrons.   The presence or absence of electrons in an atom’s current state influences the atom’s tendency to bonding.   If an atom’s outermost energy level (the valence) is full of electrons, the atom is stable and has no need to bond with another atom.   If, on the other hand, the outermost energy level is short of electrons, the atom seeks to become stable by either ionic or covalent bonding.   As atoms join together through sharing electrons, larger structures of molecules are formed.   Based on the types of atoms which join together in a molecule, one atom can attract the electrons more strongly to cause the molecule to become polarized.   Other molecules share the electrons symmetrically, resulting in molecules which are nonpolar.   Polar molecules are attracted to each other through strong intermolecular forces or bonds and as a result form the materials and liquids used every day.   Nonpolar molecules have a very weak intermolecular attraction known as van der Waals force.   The force is caused by the movement of electrons throughout the molecule, similar to a polar molecule, yet the force is constantly changing due the rapid movement of the electrons.   Therefore, the total force is much weaker than the constant attraction of polar molecules.   The properties of materials are determined by the type of intermolecular bonding as well as the shapes which the molecules join together to form.   Diamond is constructed with nothing but carbon atoms arranged in a three-dimensional lattice.   Each carbon atom has four strong covalent bonds to another surrounding carbon atom.   The entire lattice, then, has incredible strength and hardness.  

With the imaging technology available through spectroscopy and microscopy and the ability to manipulate molecules on the nano level, materials with novel properties can be formed for many useful applications.