<img src="https://d5nxst8fruw4z.cloudfront.net/atrk.gif?account=iA1Pi1a8Dy00ym" style="display:none" height="1" width="1" alt="" />

# 7.1: Introduction

Difficulty Level: At Grade Created by: CK-12

The little word, nano, has been rapidly insinuating itself into our consciousness because of its big potential. In the media, nano has captured headlines in television news channels and almost every technical and scientific journal. A number of instruments with nanometer-scale resolution made this possible. We are entering the era of nanoscience and nanotechnology—many remarkable mysteries lie ahead and several fascinating developments are forthcoming. The application of nanotechnology has enormous potential to greatly influence the world in which we live. From consumer goods, electronics, computers, information and biotechnology, to aerospace, defense, energy, environment, and medicine, all sectors of the economy are to be profoundly impacted by nanotechnology. Properties (chemical, electrical, mechanical, and optical) of materials used in these sectors changes significantly in nanoscale than their bulk form. Expected impact of nanotechnology on different sectors is illustrated in the following pie-chart, created by Lux Research—an independent research and advisory firm providing strategic advice and ongoing intelligence for emerging technologies.

Lux Research Pie Chart

## Future of Nanoscience and Nanotechnology

In 2001-02, the National Science Foundation (NSF) predicted that nanotechnology will be a \begin{align*}\ 1\end{align*} trillion global market within 10–15 years. In October 2004, Lux Research estimated market growth to \begin{align*}\ 2.6\end{align*} trillion by 2014, and in July 2008 they predicted a growth to \begin{align*}\ 3.1\end{align*} trillion by 2015, while already \begin{align*}\ 147\end{align*} billion worth of nano-enabled products were produced in 2007. It is estimated that by 2015, the scientific and technical workforce needed in nanotechnology will be greater than two million.

The following figure shows a series of technology “\begin{align*}S-\end{align*}curves.” They represent the general pattern of slow emergence of a nascent technology, followed by extremely rapid (exponential) growth, ending in a very slow growth or stagnation of the now maturing technology.

The figure shows these behaviors for cars replacing railroads for intercity transport (car growth was limited until the old horse and carriage dirt road infrastructure was replaced with roadways and cars became more reliable—growth exploded after that was accomplished). It also shows various stages of aircraft growth as the new aero technology and support infrastructure (encouraged by the government with the FAA and NACA) matured slowly at first, then grew exponentially. As of this writing (2009), nanotechnology is in its “late emerging stage” in a number of applications. The U.S. government sponsored a National Nanotechnology Initiative in 2000, which was aimed at supporting and encouraging early growth.

Time frames of Development of Technology

## What is Nano?

To understand nanoscience and nanotechnology, we have to first know what is nano? Nano means dwarf in Greek and it is a prefix in the metric scale.

Table 1. Metric Scale and Prefixes

Thus, a micrometer \begin{align*}(\mu \mathrm{m})\end{align*} is one-millionth \begin{align*}(10^{-6})\end{align*} of a meter and a nanometer \begin{align*}(nm)\end{align*} is one-billionth \begin{align*}(10^{-9})\end{align*} of a meter. Larger scales are easier to conceptualize than smaller scales. The following are some examples that provide a sense of scale (small) for milli-, micro-, and nanometer objects.

## Understanding Size

Although in the United States the standard unit of length is foot, the meter is the standard unit of length used in many other countries. Let us first examine the relationship between a foot and a meter.

\begin{align*}1 \;\mathrm{foot} = 0.3048 \;\mathrm{meter}\end{align*} or \begin{align*}1 \;\mathrm{meter} = 3.2808 \;\mathrm{feet}\end{align*}

\begin{align*}1 \;\mathrm{yard} = 0.9144 \;\mathrm{meter}\end{align*} or \begin{align*}1 \;\mathrm{meter} = 1.0936 \;\mathrm{yards}\end{align*}

\begin{align*}1 \;\mathrm{mile}= 1.609 \;\mathrm{kilometer}\end{align*} or \begin{align*}1 \;\mathrm{kilometer}= 0.6216 \;\mathrm{miles}\end{align*}

### How Small is One Millimeter (mm)?

\begin{align*}1 \;\mathrm{mm} = 0.001 \;\mathrm{meter}\end{align*}

The diameter of one dime is \begin{align*}17.91 \;\mathrm{mm}\end{align*} and the thickness is \begin{align*}1.35 \;\mathrm{mm}\end{align*}.

Dime

A CD or DVD is thinner than a dime. The diameter and thickness of a CD or DVD are \begin{align*}120 \;\mathrm{mm}\end{align*} and \begin{align*}1.2 \;\mathrm{mm}\end{align*}, respectively.

CD or DVD

We can see objects as small as \begin{align*}0.05\end{align*} millimeter \begin{align*}(mm)\end{align*}that is the limitation of the human eye. For example, the typical width of a human hair is \begin{align*}0.05 \;\mathrm{mm}\end{align*}.

Human hair

### How Small is One Micrometer (µm)?

\begin{align*}1 \ \mu \ \text{m} = 0.001 \ \text{mm}; 50 \ \mu\ \text{m} = 0.05 \ \text{mm}\end{align*}

We need a microscope to see objects smaller than \begin{align*}50 \ \mu\;\mathrm{m}=0.05 \;\mathrm{mm}\end{align*}. The most widely used microscopes are optical microscopes, which use visible light to create a magnified image of an object. The best optical microscope can magnify objects about 1000 times.

### How Small is the Smallest Thing You Can See Under a Microscope?

The smallest object that can be seen under a microscope is about:

\begin{align*}0.2-0.5 \ \mu \text{m}\ \text{(micrometer)} & = 0.0002-0.0005 \ \text{mm}\\ & = 0.0000002-0.0000005 \ \text{m}\ \text{(meter)}\\ 1 \ \mu\text{m}\ \text{(micrometer)} & = \frac{1}{1,000,000}\ \text{(meter)}\\ 10^{-6}\text{of a meter} & = \frac{1}{1000}\ \text{(millimeter)}\end{align*}

Optical-microscope

If you could split a human hair into \begin{align*}50\end{align*} separate strands, each would be about one micrometer \begin{align*}(\mu m)\end{align*} wide.

### How Small is One Nanometer (nm)?

One nanometer is

\begin{align*} 10^{-9}\text{of a meter} & = \frac{1}{1,000,000,000} \ \text{m}\,\text{(meter)}\\ \text{One-billionth of a meter} & = \frac{1}{1,000,000} \ \text{mm} \, \text{(millimeter)}\\ \text{One-millionth of a millimeter} &= \frac{1}{1,000} \ \mu\text{m} \, \text{(micrometer)} \end{align*}

If you could split a human hair into \begin{align*}50,000\end{align*} separate strands, each would be a nanometer \begin{align*}(nm)\end{align*} wide. In fact, human hairs grow by one nm every few seconds.

To see nanometer scale objects, we need an electron microscope, in which electrons are used instead of light, to see nanometer scale objects. An electron microscope can resolve objects about 1000 times smaller than an optical microscope, enabling magnifications of 1,000,000 times, without loss of detail.

## Step-by-Step Magnification

Step-by-Step Magnification

So at the nanometer scale we see molecules (a combination of different atoms connected by bonds). For example, any form of water (ice, snow, water vapor) is a combination of two hydrogen \begin{align*}(H)\end{align*} atoms and one oxygen \begin{align*}(O)\end{align*} atom, where the oxygen-hydrogen distance is about \begin{align*}0.1 \;\mathrm{nm}\end{align*}.

### Some Examples of Different Objects on the Nanoscale

Water molecule. Red and gray balls represents oxygen and hydrogen atoms, respectively.

Different Objects on the Nanoscale.

How Small is a Nano?

## Atoms and Molecules: the Building Blocks

Figure \begin{align*}7.8\end{align*} illustrates some examples that any material or object or thing (living or non-living) in this world is made from atoms. Size (radius) of atoms is about \begin{align*}0.01\end{align*} to \begin{align*}0.3 \;\mathrm{nm}\end{align*}. The human body is composed of several elements, such as carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, sulfur, calcium, iron, zinc, etc. Oxygen is the most abundant element (about \begin{align*}63\end{align*}%) in the body. The next one is carbon (\begin{align*}18\end{align*}%), followed by hydrogen (\begin{align*}10\end{align*}%), and then nitrogen (\begin{align*}3\end{align*}%). In fact, \begin{align*}99\end{align*}% of the mass of the human body is made up of the six elements oxygen \begin{align*}(O)\end{align*}, carbon \begin{align*}(C)\end{align*}, hydrogen \begin{align*}(H)\end{align*}, nitrogen \begin{align*}(N)\end{align*}, calcium \begin{align*}(Ca)\end{align*}, and phosphorus \begin{align*}(P)\end{align*}.

Nobel Prize winner Dr. Horst Störmer said that the nanoscale is more interesting than the atomic scale \begin{align*}(> 0.3 \;\mathrm{nm})\end{align*} because the nanoscale is the first point where we can assemble something—it's not until we start putting atoms together that we can make anything useful.

On the nanoscale, we can potentially assemble atoms together to make almost anything. For example, oxygen and hydrogen found in the human body is mostly as a component of water \begin{align*}(H_2O)\end{align*} molecule. Carbon, hydrogen, and oxygen are integral components of all proteins, nucleic acids (DNA and RNA), carbohydrates, and fats. The combination of all of these molecules creates the living cells of the body.

## What is Nanoscience and Nanotechnology?

The properties and functionalities of any living or non–living object come from its constituent molecule(s). Over millions of years, Mother Nature has perfected the science of manufacturing matter molecularly. Nanoscience is basically understanding science at the molecular scale. Nanoscience is both the discovery and study of novel phenomena at the nanoscale as well as the creation of new concepts to describe them.

Since the Stone Age (approximately \begin{align*}2.5\end{align*} million years ago), we have been using available materials around us to produce tools and devices for practical uses. New discoveries in science enabled us to create more application-oriented products, new devices, and electronic gadgets. Since the beginning of the 1980s, the world witnessed the development of microtechnology, a step toward miniaturization. Nanotechnology is the engineering of functional systems at the molecular scale (sizes between \begin{align*}1 - 100 \;\mathrm{nm}\end{align*}). Nanotechnology is the fabrication, characterization, production, and application of man-made devices, and systems by controlled manipulation of size and shape at a small scale that produces devices and systems with novel and superior characteristics or properties.

Table 2. Technology at a different scale.

## What Happens to Materials at the Nanoscale?

At the nanoscale, property and functionality of materials are either changed or enhanced significantly more than their bulk forms. For example, gold is a yellowish orange color when its dimension is more than \begin{align*}100 \;\mathrm{nm}\end{align*}. The color changes to green when particle size is \begin{align*}50 \;\mathrm{nm}\end{align*} and to red/ruby at \begin{align*}25 \;\mathrm{nm}\end{align*}. Similarly, silver is yellow at \begin{align*}100 \;\mathrm{nm}\end{align*}, but blue at \begin{align*}40 \;\mathrm{nm}\end{align*}. These changes in color are due to confinement of electrons in smaller areas.

Changes in properties of nanomaterials are due to greater surface area per unit mass compared with their bulk form or larger particle size. That means most of the constituent atoms are at the surface, and hence, the nanomaterials are chemically more reactive. Additionally, at the molecular scale quantum effects begin to play a vital role—affecting their optical, electrical, thermal, and magnetic behaviors.

## Why Nanoscience and Nanotechnology are Important to Us

Nanotechnology is not just the miniaturization of the electronic gadgets we use today. This \begin{align*}21^{st}\end{align*} century technology will provide a better understanding of nature's science and technology. For example, we have a deeper understanding of the underlying features at the molecular level regarding how viruses take control of normal cells within the body and spread in different conditions. For many diseases, early detection is the single most important determinant in faster and successful treatments. Besides early stage determination, we will be able to target and destroy or completely stop reactivity of molecules responsible for different diseases, including cancer, as they begin to spread in the body. A present treatment of cancer, chemotherapy, causes severe side effects as a bulk quantity of medicine is injected into the body. Nanotechnology will enable us to deliver drugs more efficiently to the exact location of cancer cells, reducing side-effects significantly. The concentration of a small molecule found in urine could reveal how advanced a patient's prostate cancer is. This recent (Jan. 2009) discovery could lead to simple, noninvasive tests for men who have the disease and might help avoid the need for biopsies. These are a few examples of nanotechnology's impact on health care.

The other aspects of nanoscience and nanotechnology are man-made nanomaterials. Over the years, scientists and technologists have developed and fabricated new materials for wider applications. The following image depicts the comparison of natural and man-made things at different sizes. Technological development at the nanoscale enables us to see and understand the underlying features of Mother Nature's science more closely.

Nature and Man-made Things in Different Scales.

The following sites have summarized some basic and pertinent information.

## A Brief History of Nanotechnology's Rapid Emergence

### Dec 29, 1959

Richard P. Feynman, a Nobel laureate physicist, made a speech (at an APS meeting at Caltech) envisioning the manipulation of materials on the nanoscale.

"The principles of physics, as far as I can see, do not speak against the possibility of maneuvering things atom by atom."

"Why cannot we write the entire \begin{align*}24\end{align*} volumes of the Encyclopedia Britannica on the head of a pin?"

Richard P. Feynman

### 1974

The term nanotechnology was coined by Tokyo Science University Professor Norio Taniguchi to describe the precision manufacturing of materials with nanometer tolerances http://en.wikipedia.org/wiki/Norio_Taniguchi.

Why did it take so long to implement nanotechnology? Because there was no tool to see and work on such a small scale.

### 1981

Gerd Binnig and Heinrich Rohrer invented the scanning tunneling microscope (STM), which can image atomic-sized objects. Electron microscopes help technology to move from micro-to nanoscale.

Heinrich Rohrer

Gerd Binnig

### 1985

C60 fullerene (also known a “buckminsterfullerenes” or “bucky balls”), a new form of carbon, was discovered by Robert F. Curl, Jr., Sir Harold W. Kroto, and Richard E. Smalley.

Fullerene, diameter . A soccer ball is a model of buckyball, but times larger.

### 1986

K. Eric Drexler, in his 1986 book Engines of Creation: The Coming Era of Nanotechnology, proposed the idea of a nanoscale "assembler," which would be able to build a copy of itself.

### 1991

Sumio Iijima, a researcher at NEC in Japan, discovered the carbon nanotube; he went on to produce an advanced, single-walled version in 1993.

Sumio Iijima

Different forms of single-wall carbon nanotubes. These are hollow tubes made from carbon atoms and their diameters vary from 0.5 to 3 nm. The longest tube synthesized so far is a few millimeters long. The discovery of fullerenes and nanotubes helped to expedite nanotechnology.

## Changes in Man-Made Technology Over the Years

### The Computer

Let us see how these metric units (mm, \begin{align*}\mu \mathrm{m}\end{align*}, and \begin{align*}nm\end{align*}) are related to technology by considering the computer as an example. The first digital computer ENIAC (dimension: \begin{align*} 2.6 \;\mathrm{m} \times 0.9 \;\mathrm{m} \times 26 \;\mathrm{m}\end{align*}, weight: about \begin{align*}54,000 \;\mathrm{lb}\end{align*}, total space: about \begin{align*}680 \;\mathrm{sq \ ft}\end{align*} or \begin{align*}63 \;\mathrm{sq}\end{align*} meter) contained \begin{align*}17,468\end{align*} vacuum tubes (acts like an on-off switch), \begin{align*}7,200\end{align*} crystal diodes (blocks electricity at certain conditions and allows it to pass when those conditions change), \begin{align*}70,000\end{align*} resistors (limits the flow of electricity), \begin{align*}10,000\end{align*} capacitors (collects electricity and releases it all in one quick burst), and around \begin{align*}5\end{align*} million hand-soldered joints.

First Digital Computer ENIAC

The size of the vacuum tube, which is a key component of the computer and other electronic devices (such as the telephone, radio, and TV), is about \begin{align*}5 - 30\end{align*} millimeter \begin{align*}(mm)\end{align*}.

Vacum tubes

The vacuum tube (invented in 1941) was replaced by much smaller millimeter scale transistors in 1955. In 1971, Intel introduced the first microprocessor, which contained about \begin{align*}2300\end{align*} transistors for use in a calculator. In the following year, Intel doubled the number of transistors in an \begin{align*}8-\end{align*}bit microprocessor designed to run computer terminals. The number of transistors in current processors, such as in the Pentium \begin{align*}4\end{align*} is more than a few million, and the size ranges between \begin{align*}0.2 \ \mu \mathrm{m}\end{align*} to \begin{align*}0.06\ \mu \mathrm{m}\end{align*} each. Presently, Intel's Duo-core chips contain \begin{align*}191\end{align*} million transistors in \begin{align*}143\end{align*} square millimeter area, and the Quad-core Itanium chip (launched in Feb. 2008) packs more than \begin{align*}2\end{align*} billion transistors in \begin{align*}65\end{align*} nanometers is almost the same size as the chip. The size of the transistor is further decreased by Taiwanese Chipmaker TSMC to \begin{align*}40 \;\mathrm{nm}\end{align*}, and recently IBM developed a \begin{align*}22.9 \;\mathrm{nm}\end{align*} chip.

Transistors

A microprocessor incorporates most or all of the functions of a central processing unit (CPU) on a single integrated circuit (IC) or chip.

Over the last 40 years, the size of the transistor, which is a key component of almost all electronic gadgets used today, was reduced in size from a millimeter to a micrometer to a nanometer. The mid-'80s to 2006 - 07 marked the period when technological development was based on micro (one-millionth of a meter) size components, and hence, termed microtechnology. Similarly, the current use of nanometer sized components (size less than \begin{align*}100 \;\mathrm{nm}\end{align*}) deem calling it nanotechnology. In the future, we will use single molecule transistors of sizes less than \begin{align*}1 \;\mathrm{nm}\end{align*}.

Single molecule transistor

### Examples of Computer Hard Disks

In 1956, IBM invented the first computer disk storage system that could store \begin{align*}5 \;\mathrm{MB}\end{align*}. It had fifty \begin{align*}24-\end{align*}inch diameter disks. The following are some images of hard disks and drives developed between 1960–1980. The weight of this hard drive is more than \begin{align*}600 \;\mathrm{lb}\end{align*}, and the diameter of the disk is \begin{align*}1\end{align*} foot. Technicians had to manually replace the disks and drives from time to time depending on usage.

Hard disk drive and hard disk

Microtechnology

In 1980, Seagate Technology introduced the first hard disk drive for personal computers. It was \begin{align*}5 \ 1/4\end{align*}" drive and held \begin{align*}5 \;\mathrm{MB}\end{align*}.

The large drive is a " full-height drive. The smaller drive is a " IDE drive. These drives also contained the disk. Currently, a ” drive is able to hold more than worth of data.

Nanotechnology

Atoms will be used in future drives and about \begin{align*}1\end{align*} million \begin{align*}GB\end{align*} worth of data may be stored in one square cm area.

Future Hard Drive

In summary, miniaturization of man-made devices significantly improves efficiency, capacity, and functionality of all electronic gadgets, and at the same time saves lots of electrical energy.

## Introduction to Electron Microscopes

Electron microscopes are the most important tools to enable us to see, manipulate, and characterize objects at the nanoscale. An electron microscope uses electrons (instead of light) to “illuminate” an object. Electron microscopes have an electron gun that emits electrons, which then strike the specimen. Conventional lenses used in optical microscopes to focus visible light do not work with electrons. Magnetic fields are used to create “lenses” that direct and focus the electrons. Because electrons are easily scattered by air molecules, the interior of an electron microscope must be sealed at a very high vacuum.

Human vision spans from \begin{align*}720 \;\mathrm{nm}\end{align*} in the red wavelengths of light to \begin{align*}400 \;\mathrm{nm}\end{align*} in the blue-violet wavelengths. The human eye cannot see electron wavelengths; therefore, we need a television-type screen or special photographic film to make electron microscope images visible to human eyes. Electrons have a much smaller wavelength than light \begin{align*}(400- 700 \;\mathrm{nm})\end{align*} and thus resolve much smaller objects. The wavelength of electrons used in electron microscopes is usually \begin{align*}5\end{align*} to \begin{align*}0.05 \;\mathrm{nm}\end{align*}.

There are two types of electron microscopes—the Scanning Electron Microscope (SEM) and the Transmission Electron Microscope (TEM). The SEM is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons. The electrons interact with the atoms that make up the sample, producing signals that contain information about the sample's surface topography, composition, and other properties such as electrical conductivity.

The TEM beam of electrons is transmitted through an ultra–thin specimen, interacting with the specimen as they pass through and then scatter providing a 2-D image of the specimen. The Scanning Transmission Electron Microscope (STEM) is a combination of SEM and TEM.

Scanning Electron Microscope

The other kind of electron microscope uses a probe that scans the surface of objects providing 3-D images of atomic networks at the surface. Extremely sharp metal points that can be as narrow as a single atom at the tip is used in scanning probe microscopes. The Scanning Tunneling Microscope (STM) is an example of this type of microscope.

How the Scanning Tunneling Microscope works.

Another type of scanning probe microscope is the Atomic Force Microscope (AFM). As the probe in an AFM moves along the surface of a sample, the electrons in the metal probe are repelled by the electron clouds of the atoms in the specimen. As the probe moves along the object, the AFM adjusts the height of the probe to keep the force on the probe constant. A sensor records the up-and-down movements of the probe, and feeds the data into a computer to construct a \begin{align*}3-D\end{align*} image of the surface of the sample.

Atomic Force Microscope

Block Diagram of Atomic Force Microscope (AFM)

AFM and STM enable us to work on atoms and design molecules the way we want by placing atoms by atoms. An excellent example is placing \begin{align*}48\end{align*} iron atoms (step-by-step) to form a quantum coral (see image at the bottom right-hand corner of Figure 11 and check out this Web site http://www.almaden.ibm.com/vis/stm/corral.html.

Applications of Atomic Force Microscope (AFM):

There are two approaches to make nanomaterials: “Top-down” and “bottom-up.” Top-down technique is as old as the Stone Age—that is cut, process, and design tools for practical purposes from large pieces of materials. This fabrication method is used to manufacture electronic circuits on the surface of silicon by etching. The most common top-down approach to fabrication of circuits involves lithographic patterning techniques using optical sources and high-energy electron beams for etching. Top-down approaches work well at the microscale, but it becomes increasingly difficult to use for nanoscale fabrication.

Building atom-by-atom and molecule-by-molecule is the philosophy of the “bottom-up” approach. This concept of a self-assembly technique comes from biological systems, where nature has harnessed chemical forces to create essentially all the structures needed for life. Different self-assembly methods have been developed for producing nanoscale materials, such as chemical vapor deposition (CVD) and molecular beam epitaxy (MBE). The basic concept of these methods is to create atoms from suitable precursors and allow them to deposit layer by layer on a substance in vacuum. In this approach highly pure nanomaterials without defects in structure can be made. Also SEM tip can be used to design and create nanostructures by placing atom by atom. This process is tedious and time consuming and is not useful for industrial purposes.

## Magic of Carbon

Carbon is one of the most abundant elements. It is not only the key element in all known life forms, but it is also present in several common materials that we use in our daily life. For example, coal, gasoline, pencil, pitch, and aromatic compounds are all carbon based. Carbon has a unique capacity to form bonds with itself and many other elements making possible to form millions of compounds.

### Graphite and Diamond

Graphite and diamond are two compounds of carbon and they have different properties. Diamond, in which each carbon is bonded to four other carbon atoms to form a three-dimensional network, is the hardest known natural material. Graphite, in which each carbon is bonded to three neighbors, is one of the softest materials. Diamond is an insulator but graphite is a good conductor of electricity. Even though graphite and diamond are the same chemically, their structures are significantly different to produce very different properties.

Diamond (left) and graphite (right) are two allotropes of carbon: pure forms of the same element that differ in structure.

### Fullerenes

Fullerenes (also known as buckyballs) and carbon nanotubes are new forms of carbons that were discovered in the late 1980s. The first fullerene reported was a hollow ball that contained sixty carbon atoms. There are \begin{align*}12\end{align*} pentagons and \begin{align*}20\end{align*} hexagons in \begin{align*}\mathrm{C}60\end{align*} and each pentagon is surrounded by \begin{align*}5\end{align*} hexagons and each hexagon is surrounded by alternating hexagons and pentagons. At present, several other cage structured fullerenes containing \begin{align*}50\end{align*} to \begin{align*}540\end{align*} carbon atoms are available. Traces of fullerene are available in nature and several chemical methods are developed to synthesize pure (\begin{align*}99.9\end{align*}%) fullerenes. Carbon nanotubes are synthesized in laboratories.

Different forms of Carbon (allotropes of carbon) : a) Diamond, b) Graphite, c) Lonsdaleite, d) C60 (Buckminsterfullerene or buckyball), e) C540, f) C70, g) Amorphous carbon, and h) single-walled carbon nanotube or buckytube.

Because of their unique structure and properties (semiconducting and electron acceptor), fullerenes can be used in different technologically based areas, such as the solar cell, trapping active molecules inside the cage, drug delivery, and bio-sensors.

### Carbon Nanotubes

Carbon nanotubes can have different forms depending on how a single hexagonal graphitic sheet is rolled to form the nanotube. Depending on their structures, carbon nanotubes can be either metallic or semiconductors. Figure 7.34 is an illustration of single-wall carbon nanotubes (SWCNT). Double-wall and multi-wall (MWCNT) nanotubes are also synthesized in the laboratory. However, synthesis results in a mixture of all kinds of nanotubes and it is hard to separate them. This has hindered some applications of individual carbon nanotubes, and current research is progressing to separate them.

However, it should be noted that nanotubes are not synthesized by rolling graphite sheet(s), tubes simply resemble rolled up graphite sheets. The following image illustrates the possibility of different forms of SWCNTs that can be related to rolling patterns of hexagonal networks of graphite sheets.

Different Forms of Single-wall Carbon Nanotube.

Single-wall Carbon Nanotubes.

This image is a nanometer carbon nanotube, filled with several cobalt nanoparticles.

It can be seen from the following tables that the carbon nanotube is lighter than aluminum but stronger than steel.

Material Elastic Modulus (GPa) Strain (%) Yield Strength (Gpa) Density \begin{align*}(\mathrm{g/cm}^3)\end{align*}
Single-wall carbon nanotube \begin{align*}1210\end{align*} \begin{align*}4\end{align*} \begin{align*}65.0\end{align*} \begin{align*}1.3\end{align*}
Multi-wall carbon nanotubes \begin{align*}1260\end{align*} \begin{align*}1.5\end{align*} \begin{align*}2.7\end{align*} \begin{align*}1.8\end{align*}
Steel \begin{align*}207\end{align*} \begin{align*}9\end{align*} \begin{align*}0.8\end{align*} \begin{align*}7.8\end{align*}
Aluminum \begin{align*}69\end{align*} \begin{align*}16\end{align*} \begin{align*}0.5\end{align*} \begin{align*}2.7\end{align*}
Titanium \begin{align*}103\end{align*} \begin{align*}15\end{align*} \begin{align*}0.9\end{align*} \begin{align*}4.5\end{align*}
Comparison of Stability, Electrical and Thermal Properties of CNTs with Other Material Used Currently
Properties Nanotubes Current Materials
Size (diameter)
SWCNT: \begin{align*}0.6 – 1.8 \;\mathrm{nm}\end{align*}
MWCNT: \begin{align*}20 – 50 \;\mathrm{nm}\end{align*}
Electron beam lithography can create lines \begin{align*}50 \;\mathrm{nm}\end{align*} wide, a few nm thick
Temperature stability Stable up to \begin{align*}2,800 ^\circ \;\mathrm{C}\end{align*} in vacuum, \begin{align*}750^\circ\;\mathrm{C} \end{align*} in air Metal wires in microchips melt at \begin{align*}600 - 1,000^{\circ}\;\mathrm{C}\end{align*}
Thermal conductivity Predicted to be as high as \begin{align*}6,000 \;\mathrm{W/m} \cdot \;\mathrm{K} \end{align*} at room temperature Nearly pure diamond transmits heat at

### Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes

Show Hide Details
Description
Tags:
Subjects: