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TEM (Transmission Electron Microscopy):

  The transmission electron microscope (TEM) forms an image by accelerating a beam of electrons that pass through the specimen. In TEM, electrons are accelerated to 100 KeV or higher (up to 1MeV), projected onto a thin specimen (less than 200 nm) by means of the condenser lens system, and penetrate the sample thickness either undeflected or deflected. The greatest advantages that TEM offers are the high magnification ranging from 50 to 106 and its ability to provide both image and diffraction information from a single sample.

            The scattering processes experienced by electrons during their passage through the specimen determine the kind of information obtained. Elastic scattering involves no energy loss and gives rise to diffraction patterns. Inelastic interactions between primary electrons and sample electrons at heterogeneities such as grain boundaries, dislocations, second phase particles, defects, density variations, etc., cause complex absorption and scattering effects, leading to a spatial variation in the intensity of the transmitted electrons. In TEM one can switch between imaging the sample and viewing its diffraction pattern by changing the strength of the intermediate lens. 

Fig: Schematic diagram of TEM

One short coming of TEM is its limited depth resolution. Electron scattering information in a TEM image originates from a three-dimensional sample, but is projected onto a two dimensional detector. Therefore, structure information along the electron beam direction is superimposed at the image plane. Although the most difficult aspect of the TEM technique is the preparation of samples, it is less so for nanomaterials.

            In addition to the capability of structural characterization and chemical analyses, TEM also has been explored for other applications in nanotechnology. Examples include the determination of melting points of nanocrystals, in which, an electron beam is used to heat up the nanocrystals and the melting points are determined by the disappearance of electron diffraction. Another example is the measurement of mechanical and electrical properties of individual nanowires and nanotubes. This technique allows a one-to-one correlation between the structure and properties of the nanowires.

STM (Scanning Tunneling Microscopy):

            The scanning tunneling microscope (STM) is a non-optical microscope that scans an electrical probe over a surface to be imaged to detect a weak electric current flowing between the tip and the surface. The STM allows scientists to visualize regions of high electron density and hence infer the position of individual atoms and molecules on the surface of a lattice. Previous methods required arduous study of diffraction patterns and required interpretation to obtain spatial lattice structures. The STM is capable of higher resolution than its somewhat newer cousin, the atomic force microscope (AFM). Both the STM and the AFM fall under the class of scanning probe microscopes. The STM can obtain images of conductive surfaces at an atomic scale 2 × 10−10 m or 0.2 nanometer, and also can be used to manipulate individual atoms, trigger chemical reactions, or reversibly produce ions by removing or adding individual electrons from atoms or molecules.

In a typical STM, a conductive tip is positioned above the surface of a sample. When the tip moves back and forth across the sample surface at very small intervals, the height of the tip is continually adjusted to keep the tunneling current constant. The tip positions are used to construct a topographic map of the surface.

For a current to occur the substance being scanned must be conductive (or semiconductive). Insulators cannot be scanned through the STM, as the electron has no available energy state to tunnel into or out of due to the band gap structure in insulators. The following figure shows the schematic diagram of STM.

Figure: schematic diagram of STM.

An extremely sharp tip usually made of metals or metal alloys, such as tungsten is mounted on to a three dimensional positioning stage made of an array of piezo electrics. Such a tip would move above the sample surface in three dimensions accurately controlled by the piezoelectric arrays. Typically the distance between the tip and the sample surface falls between 0.2 and 0.6 nm, thus a tunneling current in the scale of 0.1-10 nA is commonly generated. The scanning resolution is about 0.01 nm in XY direction and 0.002 nm in Z direction, offering true atomic resolution three dimensional image.

STM can be operated in two modes. In constant current imaging, a feed back mechanism is enabled that a constant current is maintained while a constant bias is applied between the sample and tip. As the tip scans over the sample, the vertical position of the tip is altered to maintain the constant separation. An alternate imaging mode is the constant height operation in which constant height and bias are simultaneously maintained. A variation in current results as the tip scans the sample surface because a topographic structure varies the tip-sample separation. The constant current mode produces a contrast directly related to electron charge density profiles, where as the constant height mode permits faster scan rates.  

AFM (Atomic Force Microscopy):

In spite of atomic resolution and other advantages, STM is limited to an electrically conductive surface since it is dependent on monitoring the tunneling current between the sample surface and the tip. AFM was developed as a modification of STM for dielectric materials. A variety of tip-sample interactions may be measured by an AFM, depending on the separation. At short distances, the vander walls interactions are predominant. Van der walls force consists of interactions of three components: permanent dipoles, induced dipoles and electronic polarization. Long range forces act in addition to short-range forces between the tip and sample, and become significant when the tip-sample distance increases such that the van der walls forces become negligible, examples of such forces include electrostatic attraction or repulsion, current induced or static-magnetic interactions, and capillary forces due to the condensation of water between the sample and tip.

            In AFM, the motion of a cantilever beam with an ultra small mass is measured, and the force required to move this beam through measurable distance (10-4 A0) can be as small as 10-18N.  The following figure shows a schematic diagram of AFM.

 

 

 

Figure: schematic diagram of AFM.

The instrument consists of a cantilever with a nanoscale tip, a laser pointing at the end of a cantilever, a mirror and a photodiode collecting the reflected laser beam, and a three dimensional positioning sample stage which is made of an array of piezoelectrics. Similar to STM, the images are also generated by scanning the tip across the surface. However, instead of adjusting the height of the tip to maintain a constant distance between the tip and the surface, and thus a constant tunneling current as in STM, the AFM measures the minute upward and downward deflections of the tip cantilever while maintaining a constant force of contact.

 

SPM (Scanning Probe Microscopy):

A combination of STM and AFM is also commonly referred to as scanning probe microscope (SPM). SPM is unique among imaging techniques in that it provides three-dimensional (3-D) real-space images and among other analysis techniques in that it allows spatially localized measurements of structure and properties. Under optimum conditions subatomic spatial resolution is achieved. SPM is a general term for a family of microscopes depending on the probing forces used. Two major members are STM and AFM. The limitation of STM, which is restricted to electrically conductive sample surface, is complemented by AFM, which does not require conductive sample surface. Therefore, almost any solid surface can be studied with SPM: insulators, semiconductors and conductors, magnetic, transparent and opaque materials. In addition, surface can be studied in air, in liquid, or in ultra high vacuum. In addition, sample preparation for SPM analysis is minimal.

SPM has been developed to a wide spectrum of techniques using various probe and sample surface interactions. The interaction force may be the interatomic forces between the atoms of the tip and those of a surface, short range van der walls force, or long range capillary forces, or stick-slip processes producing friction forces. Modifying the tip chemically allows various properties of the sample surface to be measured. Depending on the type of interactions between the tip and the sample surface used for the characterization, various types of SPM have been developed. Electrostatic force microscopy is based on local charges on the tip or surface, which lead to electrostatic forces between tip and sample, which allow a sample surface to be mapped, i.e., local differences in the distribution of  electric charge on a surface to be visualized. In a similar way, magnetic forces can be imagined if the tip is coated with a magnetic material, e.g. iron, that has been magnetized along the tip axis, which is Magnetic force microscopy. The tip probes the stray field of the sample and allows the magnetic structure of the sample to be determined. When the tip is functionalized as a thermocouple, temperature distribution on the sample surface can be measured, which is Scanning thermal microscopy. The capacity change between tip and sample is evaluated in Scanning capacitance microscopy. The tip can be driven in an oscillating mode to probe the elastic properties of a surface, which is referred to as Elastic modulus microscopy.

            SPM has proved its suitability in various fields of applications. First, SPM is capable of imaging the surface of all kinds of solids virtually under any kind of environment. Secondly, with various modifications of tips and operating conditions, SPM can be used to measure local chemical and physical properties of sample surface. Thirdly, SPM has been explored as a useful tool in nano-manipulation and nanolithography in fabrication and processing of nanostructures. Fourthly, SPM has also been investigated as various nanodevices, such as nanosensors and nanotwizers.

X-ray diffraction (XRD):

            XRD is a very important experimental technique that has long been used to address all issues related to the crystal structure of solids, including lattice constants and geometry, identification of unknown materials, orientation of single crystals, preferred orientation of polycrystals, defects, stresses, etc. in XRD, a collimated beam of X-rays, with a wavelength typically ranging from 0.7 to 2 A0, is incident on a specimen and is diffracted by the crystalline phases in the specimen according to Bragg’s law:

                                                λ = 2d sinө

Where d is the spacing between atomic planes in the crystalline phase and          λ is the X-ray wave length. The intensity of the diffracted X-rays is measured as a function of the diffraction angle 2ө and the specimen’s orientation. This diffraction pattern is used to identify the specimen’s crystalline phases and to measure its structural properties. XRD is nondestructive and does not require elaborate sample preparation, which partly explains the wide usage of XRD method in materials characterization.

            Diffraction peak positions are accurately measured with XRD, which makes it the best method for characterizing homogeneous and inhomogeneous strains. Homogeneous or uniform elastic strain shifts the diffraction peak positions. From the shift in peak positions, one can calculate the change in d-spacing, which is the result of the change of lattice constants under a strain. Inhomogeneous strains vary from crystallite to crystallite or within a single crystallite and this causes a broadening of the diffraction peaks that increase with sinө. Peak broadening is also caused by the finite size of crystallites, but here the broadening is independent of sinө. When both crystallite size and inhomogeneous strain contribute to the peak width, these can be separately determined by careful analysis of peak shapes.

            If there is no inhomogeneous strain, the crystallite size, D, can be estimated from the peak width with the Scherrer’s formula:

Where λ is the X-ray wave length, B is the full width of height maximum of a diffraction peak,  is the diffraction angle, and K is the Scherrer’s constant of the order of unity for usual crystal. However, one should be altered to the fact that nanoparticles often form twinned structures: therefore, Scherrer’s formula may produce results different from the true particle sizes. In addition, X-ray diffraction only provides the collective information of the particle sizes and usually requires a sizeable amount of powder. It should be noted that since the estimation would work only for very small particles, this technique is very useful in characterizing nanoparticles. Similarly, the film thickness of epitaxial and highly textured thin films can also be estimated with XRD.

            One of the disadvantages of XRD, compared to electron diffraction, is the low intensity of diffracted X-rays, particularly for low-Z materials. XRD is more sensitive to high – Z materials, and for low Z-materials, neutron or electron diffraction is more suitable. Typical intensities for electron diffraction are ~108 times larger than for XRD.  Because of small diffraction intensities, XRD generally requires large specimens and the information acquired is an average over a large amount of material. The following figure shows the powder XRD spectra of a series of nanoparticles with different sizes.

 

Special nanomaterials:

Introduction:

In the previous chapters, we have introduced the fundamentals and general methods for the synthesis and fabrication of various nanostructures and nanomaterials including nanoparticles, nanowires and thin films. How ever, there are a number of important nanomaterials not included, since their synthesis is unique and difficult to group into previous chapters. Examples of such nanomaterials are carbon fullerenes and nanotubes. In addition, bulk materials with nanosized building blocks, such as nanograined ceramics and nanocomposites have not been discussed so far. In this chapter, we will discuss the synthesis of these special nanomaterials. Most of these nanomaterials are unique, do not exist in nature, and are truly “man-made” relatively recently, therefore, a brief introduction to the materials, such as their peculiar structures and properties, has also been included in this chapter.

            Carbon is a unique material, and can be a good metallic conductor in the form of graphite, a wide band gap semiconductor in the form of diamond, or a polymer when reacted with hydrogen. Carbon provides examples of materials showing the entire regime of intrinsic nanometer scaled structures from fullerenes, which are zero-dimensional nanoparticles, to carbon nanotubes, one-dimensional nanowires to graphite, a two dimensional layered anisotropic material, to fullerene solids, a three dimensional bulk materials with the fullerene molecules as the fundamental building block of the crystalline phase.

Carbon fullerenes:

Carbon fullerene commonly refers to a molecule with 60 carbon atoms, C60, and with an icosahedral symmetry, but also includes larger molecular weight fullerenes Cn (n>60). Examples of larger molecular weight fullerenes are C70, C76, C78, C80, and higher mass fullerenes, which possess different geometric structures, e.g. C70 has a rugby-ball shaped symmetry. The following figure shows the structure and geometry of C60 molecule.

Figure: The icosahedral C60 molecule.

The name of fullerene was given to this family of carbon molecules because of the resemblance of these molecules to the geodesic dome designed and built by R. Buckminster Fuller, where as the name of Buckminster Fullerene or buckyball was specifically given to the C60 molecules, which are most widely studied in the fullerene family and deserve a little more discussion on its structure and properties.

Properties: The 60 carbon atoms in C60 are located at the vertices of a regular truncated icosahedron and every carbon site on C60 is equivalent to every other site. The average nearest neighbor C-C distant in C60 (1.44 A0) is almost identical to that in graphite (1.42A0). Each carbon atom in C60 is trigonally bonded to other carbon atoms, the same as that in graphite, and most of the faces on the regular truncated isosahedron are hexagons. There are 20 hexagonal faces and 12 additional pentagonal faces in each C60 molecule, which has a molecule diameter of 7.10A0. Some selective properties of C60 molecule were given in the following table.         

Property

value

Cage diameter

0.7 nm

Vander walls diameter

1.0 nm

Bond distances: five-six bonds

 

 

0.1404 nm

 

Bond distances: six-six bonds

 

 

0.1448 nm

 

symmetry

isosahedral

Electron affinity

2.65 ev

First ionization potential

7.58 ev

Cohesive energy

7.4 ev/atom

Synthesis:

There are four methods for the production of fullerenes. They are radio frequency thermal plasma method, laser vaporization, RF-inductive coupled plasma discharge method and flame combustion method. 

Fullerenes are usually synthesized by using an arc discharge (flame combustion method) between graphite electrodes in approximately 200 torr of He gas. The heat generated at the contact point between the electrodes evaporates carbon to form soot and fullerenes, which condense on the water cooled walls of the reactor. This discharge produces carbon soot that contains up to ~15% fullerenes: C60 (~13%) and C70 (~2%). The fullerenes are next separated from the soot, according to their mass, by use of liquid chromatography and using a solvent such as toluene. However, there is no definite understanding of the growth mechanism of the fullerenes.  The following figure shows the schematic diagram of fullerene soot production chamber.

Applications: Speculation and some hard work on potential applications began almost immediately after the discovery of buckyballs. There are some potential applications of fullerenes as listed below.

1. As fullerenes are very large graphitic systems, they can easily accommodate extra electrons. When we add three electrons to C60 we get ionic solids of the general formula A3C60, where A is any metal in Group I (lithium, sodium, potassium, rubidium, cesium). These materials are actually metals, and display superconductivity at some what low temperatures. Current research is aimed at getting the maximum superconducting temperature (or Tc) to higher values.

2. C60 is just the right size to fit into the active cavity of HIV Protease, an enzyme important to the activity of the virus which causes AIDS. Cramming a buckyball into the active cavity would deactivate the enzyme and kill the virus. Ways of getting the molecule to the enzyme are under investigation

3. Possible applications of interest to industry include optical devices; chemical sensors        and chemical separation devices; production of diamonds and carbides as cutting tools or hardening agents; batteries and other electrochemical applications, including hydrogen storage media; drug delivery systems and other medical applications; polymers, such as new plastics; and catalysts. Catalysts, in fact, appear to be a natural application for fullerenes, given their combination of rugged structure and high reactivity. Experiments suggest that fullerenes which incorporate alkali metals possess catalytic properties resembling those of platinum.

4. The C60 molecule can also absorb large numbers of hydrogen atoms--almost one hydrogen for each carbon--without disrupting the buckyball structure. This property suggests that fullerenes may be a better storage medium for hydrogen than metal hydrides, the best current material, and hence possibly a key factor in the development of new batteries and even of non-polluting automobiles based on fuel cells.

5. A thin layer of the C70 fullerene, when deposited on a silicon chip, seems to provide a vastly improved template for growing thin films of diamond.

Carbon nanotubes:

A promising group of nanostructured materials is the nanotubes, which are currently fabricated from various materials such as boron nitride, molybdenum, carbon (carbon nanotube), etc. However, at the moment, carbon nanotubes seem to be superior and most important due to their unique structure with interesting properties, which suit them to a tremendously diverse range of applications in micro or nanoscale electronics, biomedical devices, nanocomposites, gas storage media, scanning probe tips, etc.

 

Definition:

Carbon nanotubes are a new form of carbon made by rolling up a single graphite sheet to a narrow but long tube closed at both sides by two hemispheres (1/2 section of fullerene carbon) like end caps.

            In 1991, while experimenting on fullerene and looking into soot residues sumio lijima invented two types of nanotubes namely single walled carbon nanotubes (SWNTs) and multi walled carbon nanotubes (MWNTs). SWNT consists only of a single graphene sheet with one atomic layer in thickness, while MWNT is formed from 2 to several tens of graphene sheets arranged concentrically into tube structures. They are promising one-dimensional periodic structure along the axis of the tube with high aspect ratio (length/diameter).

 

Properties:

The following table shows selected electrical and mechanical properties of carbon nanotubes.

Characteristics

Measure

Electrical conductivity

 

Metallic or semi conducting

 

Electrical transport

 

Ballistic, no scattering

 

Energy gap (semi conducting)

 

       E g (ev)=1/d (nm)

Maximum current density

 

1010 A/cm2

 

Maximum strain

 

0.11% at 1 V

 

Thermal conductivity

 

6 KW/Km

 

Diameter

 

1-100 nm

 

Length

 

Up to millimeters

 

Gravimetric surface

 

>1500 m2/g

 

E-modulus

1000 Gpa, harder than steel

         Nanotubes can be either electrically conductive or semi conductive, depending on their helicity.

         These one-dimensional fibers exhibit electrical conductivity as high as copper, thermal conductivity as high as diamond.

         Strength 100 times greater than steel at one sixth the weight, and high strain to failure.

         Current length limits are about one millimeter.

Synthesis (production of carbon nanotubes):

The growth of carbon nanotubes during synthesis and production is believed to commence from the recombination of carbon atoms split by heat from its precursor. Although a number of newer production techniques are being invented, three main methods are the laser ablation, electric arc discharge and the chemical vapor deposition. Chemical vapor deposition is becoming very popular because of its potential for scale up production.

 

Chemical vapor deposition:

In this technique, carbon nano tubes grow from the decomposition of hydrocarbons at temperature range of 500 to 12000C. They can grow on substrates such as carbon, quartz, silicon, etc or on floating fine catalyst particles, e.g. Fe, Ni, Co, etc from numerous hydrocarbons e.g. benzene, xylene, natural gas, acetylene, to mention but few.

 

 

 

The above figure shows the schematic diagram of a typical catalytic chemical vapor deposition system. It is equipped with a horizontal tubular furnace as the reactor. The tube is made of quartz, 30 mm in diameter and 1000 mm in length. Ferrocene and Benzene vapors acts as the catalyst (Fe) and carbon atom precursors respectively were transported either by argon, hydrogen or mixture of both into the reaction chamber, and decomposed into respective ions of Fe and carbon atoms, resulting into carbon nanostructures. The growth of the nanostructures occurred in either the heating zone, before or after the heating zone, which is normally operated between 5000C and 11500C for about 30 min. 200ml/min of hydrogen is used to cool the reactor.

Arc discharge:

The arc discharge method produces a number of carbon nanostructures such as fullerenes, whiskers, soot and highly graphitized carbon nanotubes from high temperature plasma that approaches 37000C. The first ever produced nanotube was fabricated with the DC arc discharge method between two carbon electrodes, anode and the cathode in a noble gas (helium or argon) environment. Schematic representation of a typical arc discharge unit is presented in figure below

 

 

 

Figure: Schematic of Arc discharge method.

Relatively large scale yield of carbon nanotubes of about 75% was produced by Ebbesen and Ajayan with diameter between 2 to 30nm and length 1µm deposited on the cathode at 100 to 500 Torr He and about 18 V DC. It has conveniently been used to produce both SWNTs and MWNTs as revealed by Transmission Electron Microscope (TEM) analysis. Typical nanotubes deposition rate is around 1mm/min and the incorporation of transition metals such as Co, Ni or Fe into the electrodes as catalyst favors nanotubes formation against other nanoparticles, and low operating temperature. The arc discharge unit must be provided with cooling mechanism whether catalyst is used or not, because overheating would not only results into safety hazards, but also into coalescence of the nanotube structure.

Laser ablation:

Laser ablation technique involves the use of laser beam to vaporize a target of a mixture of graphite and metal catalyst, such as cobalt or nickel at temperature approximately 12000C in a flow of controlled inert gas (argon) and pressure, where the nanotube deposits are recovered at a water cooled collector at much lower and convenient temperature. This method was used in early days to produce ropes of SWNTs with remarkably uniform narrow diameters ranging from 5-20 nm, and high yield with graphite conversion grater than 70-90%.

The bundles entangled into a 2-D triangular lattice via the van der walls bonding to achieve lattice constant equal to 1.7 nm. The metal atom (catalyst) due to its high electronegativity, deprived the growth of fullerenes and thus a selective growths of carbon nanotubes with open ends were obtained. Changing the reaction temperature can control the tubes diameters, while the growth conditions may be maintained over a higher volume and time, when two laser pulses are employed.

            However, by the virtue of relative operational complexity, the laser ablation method appears to be economically disadvantageous, which in effect hampers its scale up potentials as compared to the CVD method. The following figure shows the schematic of laser ablation method.

Current and future applications:

Currently, carbon Nanotubes are extending our ability to fabricate devices such as Molecular probes, Pipes, Wires, Bearings, springs, Gears, Pumps, Molecular transistors. In future we can find some more applications such as Field emitters, Building blocks for bottom-up electronics, Smaller, lighter weight components for next generation spacecraft and also enable large quantities of hydrogen to be stored in small low pressure tanks.