Glossary of terms used in Nanotubes module

Glossary for Nanotubes

Absolute Zero The lowest possible temperature and energy of a thermodynamic system. It is equal to 0 Kelvin, -273.15° Celsius, or -459.67° F. Click here to see a primer on the three temperature scales.

Angular Momentum Generally denoted by L, the angular momentum about a given point equals the cross product Dr. Shirley Ann Jackson demonstrates conservation of angular momentum.

Dr. Shirley Ann Jackson demonstrates conservation of angular momentum.

of distance r from that point and linear momentum p: L = r x p.

The figure on the left shows Rensselaer president Dr. Shirley Ann Jackson using a modified bicycle wheel to demonstrate how angular momentum is always conserved (never changes).

It is assumed that the user of these materials has some familiarity with angular momentum. If not, see this link (click "Visit Site") for a ThinkQuest page explaining angular momentum.

(Energy) Band The continuous range of allowed energies for electrons in a solid. Individual atoms can have only certain quantized energies. As atoms bond to form solids, each atomic energy level "spreads" to accommodate the shifted levels of adjacent atoms. Atoms in the solid can occupy any energy in one of the newly-formed "bands". This page of the semiconductor module provides more detail and animations depicting the formation of energy bands.

Bit Short for "binary digit," a bit is a single piece of information. It can be either 1 or 0.

Capacitor A device for storing electric energy. Typically, capacitors consist of two parallel conducting plates, separated by an insulator. Electric charge can be stored on the plates.

Carbon nanotube

A tiny tubular structure made of carbon atoms arranged in a hexagonal structure (see image). Nanotubes are typically a few nanometers across, and they come in a variety of lengths. The structure of nanotubes is so stable, they can in principle grow to any length desired, even lengths on the order of kilometers. In practice, nanotubes are produced with lengths ranging from a few nanometers to hundreds of nanometers. (Image courtesy of Mitch Mailman and Saroj Nayak.)

The structure of carbon atoms forming a nanotube: hexagons connected to make a cylinder.

Cathode Ray Tube (CRT) A tube that directs a beam of electrons (also know as cathode rays), generally toward a phosphorescent screen that lights up where the electrons strike. CRTs have been around for over a century and were the fundamental technology for television sets, computer screens, and other types of screens. While flat screens based on various other technologies are gaining popularity and becoming more affordable, most television sets, and many computer monitors, still rely upon CRTs. Click here to go to the Wikipedia page on CRTs.

Charge Carrier A particle having electric charge that can move freely through the material. In conductors, electrons are free to move, so they are the charge carriers. Holes are the charge carriers in p-type semiconductor. Positive or negative ions can be charge carriers in some liquids.

Chip

Click on this picture of an etched silicon wafer to see an enlarged version.

A collection of millions of integrated (interconnected) semiconductor devices that performs a particular function. During the fabrication process, thousands hundreds of chips will be simultaneously created on a single semiconductor "wafer," such as the one in the picture to the left. The wafer in the picture contains ~450 memory chips, used for electric storage in RAM. (If you click on the picture, you can see an enlarged version.) Electronic storage is discussed in this module of the ScIT materials.

Conduction Band

The unfilled top energy band in a solid. Since this band is not filled, electrons with energies in this band can move easily through the solid, creating an electric current. The energy diagram to the right follows this author's convention of representing electrons in the conduction band by green circles. The blue circles represent electrons in filled (or mostly filled) bands, such as the valence band in the image to the right.

Conduction Electron

An electron that is free to move within a solid. The motion of these electrons can give rise to the conduction of electricity by creating an electric current through the solid . The energy diagram of an intrinsic semiconductor found to the right follows this author's convention of representing conduction electrons by green circles. The blue circles represent tightly bound electrons that do not significantly contribute to conduction through a solid.

Conductor A material with low resistivity used for contacts and interconnects in semiconductor processing. Conductors have a partially filled In a conductor, the valence band is only partly full, so the conduction band is the valence band.

In a conductor, the valence band is only partly full, so the conduction band is the valence band.

valence band, through which electrons can move freely, as shown in the energy diagram to the left. Thus, in a conductor, the conduction band is the same as the valence band, and the charge carriers are primarily electrons. In keeping with this author's convention, the electrons in this partially-filled conduction band are represented by green circles.

(Electrical) Conductivity (s) A measure of how freely current can flow through a material. Copper, with its high conductivity of 5.95 x 10⁷ W^-1m^-1, conducts electric current more freely than does aluminum, with its slightly lower conductivity of 3.77 x 10⁷ W^-1m^-1. Conductivity is the inverse of resistivityr : s = 1/r.

Coulomb (C) The SI unit of electric charge. One coulomb is a fairly large amount of charge, equaling the charge of 6.25 x 10¹⁸ protons.

Cross-Sectional Area (A)

Cross-sectional areas of a narrowing pipe.

The (2-dimensional) area formed when one "slices" through a (3-dimensional) solid. When discussing electric current, we typically take a cross section that is perpendicular to the direction of current flow, like the two cross sections of the pipe shown to the left. Link here for a more technical definition from Wolfram's MathWorld.

Doping The process of introducing impurities into a substance to enhance or control its properties. For example, silicon is often doped with gallium or with phosphorus. Both increase the conductivity of silicon; gallium increases the concentration of holes in the silicon, and phosphorus increases the concentration of conduction electrons. Doping not only increases the conductivity but it can be used to produce diodes (including LEDs), transistors, and many other devices. The discussion of doping starts on this page of the semiconductor module.

(Electric) Current (I) The rate at which electric charge flows past a given point. The convention in physics is to discuss the flow of positive charges, even though we now know that the motion of (negatively-charged) electrons is what creates a current in conductors. According to this convention, if a beam of electrons traveled from right to left across your screen, we would say the current flows from left to right. The SI unit is the Ampere, or Amp; one Amp is equal to one Coulomb of charge per second of time. For example, if particles carrying 5 Coulombs of net electric charge flowed into a light bulb each second, we would say that the current through the bulb was 5 Amps. For a good introduction to current and other circuit properties, try this page of the All About Circuits site.

Electric memory Temporary storage devices in a computer. Electric memory devices are generally combinations of capacitors and transistors, as discussed in the electric memory module. Electric memory devices can be built into chips, and are more convenient to the processor than are magnetic storage devices (like the hard drive) or optical storage devices (like the CD-ROM). Electric memory is not well-suited for long-term storage, since loss of power will erase it.

Electron A fundamental particle with negative charge found in atoms. It has a mass of 9.11 x 10^-31 kilograms and a charge of - 1.6 x 10^-19 Coulombs. Conduction occurs primarily from the movement of electrons through materials. Click this link to go to the Particle Adventure webpage for more information on electrons.

Energy Energy comes in many different forms, such as "kinetic energy" (energy of motion), heat, and even mass. Energy can never be created or destroyed; it can merely change from one form to another. These materials assume the reader has some prior knowledge of energy as it is defined in science. If you are not confident in your prior knowledge, you can access Glenbrook High's Physics Classroom pages on energy, by clicking here. If you only want an overview, try this nice site by the Danish Wind Industry Association.

Energy Diagram Energy states in a silicon atom: two electrons fit in the n=1 energy level and eight fit in the n=2 level, leaving four electrons to partly fill the n=3 energy level.

Energy states in a silicon atom: two electrons fit in the n=1 energy level and eight fit in the n=2 level, leaving four electrons to partly fill the n=3 energy level.

A plot showing energies on the vertical axis, typically used to illustrate the specific quantized energies allowable for electrons (Disclaimer) in atoms and solids. The energy diagram for a silicon atom shown on the left indicates the fourteen electrons in their lowest states. (The Pauli exclusion principle determines how many electrons can "fit" into each value of energy.) The band diagram on the right, shown for a pure semiconductor such as silicon, Band diagram of a semiconductor; some electrons have jumped from the full valence band to the emptly conduction band, where they can move through the solid.

Band diagram of a semiconductor; some electrons have jumped from the full valence band to the emptly conduction band, where they can move through the solid.

shows electrons populating the energy bands of a solid. Energy is again the vertical axis; the horizontal axis represents location in one dimension. Both diagrams follow this author's color convention: blue electrons are tightly bound in filled states or bands and not likely to move or interact; green electrons are more loosely bound and participate in bonding (between atoms) or conduction (within a solid).

(Energy) State A complete set of variable values that a particle can exhibit. For example, an electron in a hydrogen atom can be described by its location, its energy (Disclaimer), and its angular momentum. The collection of these properties constitutes the "state" of the electron. According to quantum theory, an electron in an atom can take only certain, quantized, values of energy and angular momentum, so the number of states in an atom is limited.

(Pauli) Exclusion Principle First proposed by Wolfgang Pauli to explain the arrangement of electrons in atoms, the Exclusion Principle asserts that no two electrons can be in the same state. In other words, two electrons in the same atom cannot have the same set of values for energy (Disclaimer) and other physical qualities. We now know that the Exclusion Principle applies not just to electrons, but to an entire class of particles called "fermions", which includes protons and neutrons.Click here to go to the Particle Adventure's website, or click here to go to the Physics 2000 page explaining the Exclusion Principle.

Focus (of Ellipse)

The distances r1 and r2 from the foci of an ellipse to a point on the ellipse will always add to give the same value.

One of two points (foci) within an ellipse that define its shape. At every point on the ellipse, adding r₁ (the distance from one focus to point) to r₂(the distance from the other focus to the point) will yield the same number, equal to 2a on the figure to the right. a is called the "semimajor axis" since it is half the distance of the longer axis. For more information about ellipses, try this website from the Punahou School in Hawaii.

Ground State The energy state of an atom, or of a solid, when its total energy equals the minimum possible energy for that atom or solid. For solids, this only occurs at a temperature of absolute zero. Individual atoms may be in their ground state at non-zero temperatures, but they will not stay there indefinitely. Instead, they can occasionally absorb thermal energy and move to an "excited" state for a while before they decay back to the ground state. For our purposes, we are primarily interested in the energies of electrons in atoms and solids (Disclaimer), so when we refer to "ground state" we mean all the electrons having their lowest possible energy.

Hole Essentially the absence of an electron in an otherwise filled energy band, a hole can be treated as a positively-charged particle moving through the valence band of a solid. Holes are explained on this page of the semiconductor module.

Insulator A material with very high resistivity (low conductivity) often used to prevent contact between conductors. The poor conductivity is due to the completely full valance band, illustrated in the energy diagram to the right. Energy bands of an insulator; one electron has jumped from the full valence band to the emptly conduction band.

Energy bands of an insulator; one electron has jumped from the full valence band to the emptly conduction band.

An insulator has no charge carriers at absolute zero and very few charge carriers at room temperature. The energy diagram to the right follows this author's convention of representing conduction electrons by green circles. The blue circles represent tightly bound electrons that do not significantly contribute to conduction through a solid. The absence of a blue circle is a hole, which can contribute to conduction, as described on this page of the semiconductor module.

Interconnect A strip of conductor on a computer chip that connects circuit elements such as transistors and resistors to each other.

Joule The SI unit of energy, one Joule is a Newton-meter, or a kg m²/s².

(Linear) Momentum Generally denoted by p, linear momentum is the mass m of an object times its velocity v: p = mv. Since velocity has direction, so does momentum. It is conserved (doesn't change) in the absence of forces. It is assumed that the user of these materials has some familiarity with momentum. If not, see this link to the Glenbrook High's Physics Classroom for a good introduction to the topic.

Moore's Law

Not really a law, but a prediction made by Gordon Moore of Intel in 1965. Moore predicted that the number of transistors that could be fit onto a single chip would double every 18 to 24 months.

Multi-walled nanotube

A nanotube with more than one layer, multi-walled nanotubes can consist of either concentric capsules, like the figure on the far right, or of a single sheet rolled up, like the figure on the left. A single-walled nanotube, a multiwalled like rolled carpet nanotube, and a multiwall of concentric cylinders nanotube. Multi-walled nanotubes are easier to produce than single-walled nanotubes, but they do not exhibit all of the desired properties of the single-walled variety. Scientists are looking for practical ways to mass-produce single-walled nanotubes while continuing to explore applications and production techniques of multi-walled tubes.

n-type Semiconductor Consists of a semiconductor, such as silicon, which has been doped with atoms of an "donor" element, such as phosphorus. The additional electronsof the donor atoms provide filled energy states in the band gap, just below the conduction band. An n-type semiconductor is doped to add electrons in energy levels right below the conduction band.

An n-type semiconductor is doped to add electrons in energy levels right below the conduction band.

Electrons from these "extra" states can easily move into the conduction band and become conduction electrons, as shown in the energy diagram to the right. Replacing even one in every million silicon atoms with a phosphorus atom can increase the conductivity of the solid by a factor of five million. Because the density of conduction electrons provided by the donor is much larger than the density of holes contributed by the semiconductor atoms, conduction in this material is primarily due to the motion of (negatively-charged) electrons. Thus it is called an n-type semiconductor. n-type semiconductors are discussed on this page of the semiconductor module.

Nanometer

10^-9 meters (0.000000001 meter, or one trillionth of a meter). A nanometer is to a meter what a penny is to ten million dollars.

NOT gate

A circuit element that accepts a single input bit and outputs the opposite value. For example, if the input of a NOT gate is a 1 (signal "on"), the output is a 0 (signal "off"). And if the input is a 0, the output is a 1.

Ohmic (Materials) Materials obeying Ohm's Law. Ohmic materials have resistances that do not change over a wide range of voltage and currents. For an ohmic device, a graph of voltage across the device vs the current through that device will yield a straight line, with constant slope equal to the resistance of the device.

Ohm's Law An observation (first made by Georg Simon Ohm) that applies to certain materials or devices, called ohmic materials or devices. States that many materials have resistances that are independent of voltage and current under regular operating conditions. Ohm's Law is NOT the statement V = IR, as many believe. That statement is instead the definition of resistance. Ohm's Law instead says that, for many materials under a wide range of conditions, the voltage V and current I are linearly related, which implies a resistance R independent of V and I. To read more about Ohm's Law, try this page, of the ohmslaw.com website.

p-type Semiconductor Consists of a semiconductor, such as silicon, which has been doped with atoms of an "acceptor" element, such as gallium. A p-type semiconductor is doped to add extra empty energy states right above the valence band.

A p-type semiconductor is doped to add extra empty energy states right above the valence band.

The unfilled states in the acceptor atoms provide empty energy states in the band gap of the semiconductor, just above the valence band. Electrons can move into these "extra" states, leaving behind holes in the valence band, as shown in the energy diagram to the right. Replacing even one in every million silicon atoms with a gallium atom can increase the conductivity of the solid by a factor of five million. Because the density of holes provided by the acceptor is much larger than the density of conduction electrons contributed by the semiconductor atoms, conduction in this material is primarily due to the motion of (positively-charged) holes. Thus it is called a p-type semiconductor. p-type semiconductors are discussed on this page of the semiconductor module.

Perigee

Point of closest approach in an orbit. The Moon's orbit is not perfectly circular, but takes the shape of an ellipse, with the Earth at a focus of the ellipse. The distance between the Moon and the Earth ranges from 409,000 km (240,000 mi) at "apogee", the point furthest from the Earth, to 365,000 km (227,000 mi) at perigee. These numbers, along with much more about the moon's orbit, can be found at this infoplease web page.

Periodic Table A table of all known elements, sorted into columns by their chemical properties and into rows by their relative masses within the columns. Mendeleev was the first to organize elements in such a manner, and he successfully predicted the existence of elements that had not been previously detected or expected. Click here for a nice on-line table coded by Michael Dayah. Another nice on-line periodic table can be found at this site by webelements.com. For a description of how Mendeleev organized the elements, try this Physics 2000 page.

Phosphorescent Phosphorescent materials emit visible light after they are struck by a particular type of particle or radiation. For example, CRTs, such as are in traditional television sets, bombard a screen with electrons. When an electron strikes the screen, it is absorbed. The screen then emits the excess energy in the form of visible light.

Quantized Having only certain, discrete values. For example, whole numbers are quantized. They can only take the values 0, 1, 2, 3, etc. and will never fall between those discrete values. Decimals, on the other hand, can take any value and thus are continuous rather than being quantized. (Disclaimer)

RAM Acronym for Random Access Memory, RAM is temporary storage in a computer. RAM uses electric memory devices, which are closer to the processors than magnetic storage (i.e., hard drive) or optical storage (i.e., CD-ROM) would be. This allows faster access for information the computer needs for current processes.

Resistance (R) The ratio between the voltage V applied to a device and the electric current I that flows through it:

R = V/I.

Certain materials (called ohmic materials) have resistance that is independent of voltage and current. Typically, the word "resistor" refers to an ohmic device in a circuit. For a good introduction to resistance and other circuit properties, try this page of the All About Circiuts site.

Resistivity (r) The "part" of the resistance of an object independent of the geometry of the object. Resistivity depends on the type of material used and on the temperature of the object. For a homogeneous solid, resistivity r is related to resistance R in the following manner:

r = R A / L,

where A is the cross-sectional area of the solid, and L is its length, as shown to the right. Resistivity is the inverse of conductivitys:

r = 1/s. Copper, with its low resistivity of 1.68 x 10^-8 Wm conducts electric current more freely than does aluminum, with its slightly higher resistivity of
2.65 x 10^-8 Wm.

A cut-away veiw of a wire showing length and cross-sectional area.

Semiconductor A subclass of insulators, semiconductors are materials with conductivity that can be controlled through methods such as doping or changing the temperature. Like all insulators, they have a valence band that is completely full in the ground state. A semiconductor has no charge carriers at absolute zero; "pure," or "intrinsic," semiconductors have very few charge carriers even at room temperature. Conductivity can be increased through doping, creating either p-type semiconductors or n-type semiconductors.

Single-walled nanotubes A single-walled nanotube, a multiwalled like rolled carpet nanotube, and a multiwall of concentric cylinders nanotube.

A nanotube composed of a single layer, single-walled nanotubes exhibit a variety of desirable properties. Single-walled nanotubes are much more difficult to produce than multi-walled nanotubes, but their unique properties promise exciting applications. Scientists are looking for practical ways to mass-produce single-walled nanotubes while continuing to explore applications and production techniques of multi-walled tubes.

Thermal conductivity A measure of how freely heat can flow through a material. Thermal conductivity is the ratio between the rate R of heat flow over a boundary to the area A of an temperature change DT across the boundary: s _T = R / (DT * A). Diamond has an incredibly high thermal conductivity that can reach 2600 W K^-1m^-1. Metals, which are also considered good thermal conductors, typically have thermal conductivites ranging from 200 to 500 W K^-1m^-1. The thermal conductivity of nanotubes has been measured as high as 6600 W K^-1m^-1, according to this paper which appeared in Physics Review Letters in 2000. For more information about thermal conductivity and values for other substances, see this Wikipedia page.

Transistor An npn transistor with three regions of doped semiconductor (n, p, then n), the gate terminal, and its insulation.

An electronic switch. Transistors allow a (relatively) large amount of current to flow when a (relatively) small voltage is applied, just like a light switch can provide a large amount of electric energy to a lamp when a small amount of mechanical energy is expended to flip the switch. Transistors in modern electronics are made from layered p- and n- type semiconductors. Click here to go to the start of the module on transistors.

Tunneling A process predicted by quantum theory in which a particle crosses to the other side of a barrier it shouldn't be able to go over. For example, consider an electron approaching a region of negative charge that repels the electron. According to classical physics, the electron does not have enough energy to pass through this region - it will slow down, stop, and be accelerated back the way it came. Quantum physics, however, allows a small percentage of such electrons to make it past the region of negative charge, effectively "tunneling through" the barrier. Such tunneling has been observed in nature; many technologies such as the scanning tunneling microscope are based upon this behavior. For more information on tunneling, go to this Wikipedia page.

Vacuum The absence of matter. Outer space is very close to a pure vacuum, with an average of .07 particles found in every cubic meter.

Vacuum annealing A process by which a surface is heated in a vacuum, vacuum annealing is commonly used in the semiconductor industry and in related fields of research. Semiconductors such as silicon are heated to allow absorption of dopants or other impurities. The process is carried out in a vacuum so the only impurities available to be absorbed are those that have been provided by the experimenter. For more information, see this page of About.com.

Valence Band The outermost energy band that contains electrons when a solid is in the ground state. An intrinsic semiconductor (or insulator) in its ground state will have a completely filled valence band, while the conduction band above the valence band is completely empty. The valence band of a solid is the outermost energy band occupied at absolute zero.

The valence band of a solid is the outermost energy band occupied at absolute zero.

The energy diagram to the right depicts a semiconductor in which some electrons have gained energy to jump to the conduction band, leaving behind hole in the valence band. (Disclaimer) These holes can move through the nearly-full valence band, just as the electrons can move through the nearly-empty conduction band. The image follows this author's convention of representing electrons bound in the valence band by blue circles. The green circles represent electrons that are free to move, such as the ones in the conduction band in the image to the right.

Voltage (V), or Potential Difference The change in energy per unit of electric charge, measured in the SI unit of Volt (V). For example, a 1.0 Volt battery increases the energy of each Coulomb of charge by one Joule. For a good introduction to voltage and other circuit properties, try this page of the All About Circuits site.