Answers for Question bank for Mid-Sem in Nanotechnology

The answers in this document are taken from Slides taught in the class and internet. Some of the

answers are very lengthy. Such answers can be cut-short depending on the requirement. I don’t know

the appropriate answer(s) for the last three questions of this Question Bank.

1. Define nanotechnology. Give two examples each of materials naturally occurring at

nanoscale and man made nanomaterials.

The design, characterization, production, and application of structures, devices, and

systems by controlled manipulation of size and shape at the nanometer scale (atomic,

molecular, and macromolecular scale) that produces structures, devices, and systems with

at least one novel/superior characteristic or property is known as Nanotechnology.

Example Naturally Occurring: DNA, ATP Synthase Man Made Nanomaterials: carbon nanotubes, nanocomposite structures or nanoparticles of a particular substance

2. Explain different types of bondings with example.

A bond is formed when electrons from two atoms interact with each other and their atoms

become joined. The electrons that interact with each other are VALENCE ELECTRONS,

the ones that reside in the outermost electron shell of an atom.

There are two main types of bonding discussed here. A COVALENT BOND results

when two atoms "share" valence electrons between them. An IONIC BOND occurs when

one atom gains a valence electron from a different atom, forming a negative ion

(ANION) and a positive ion (CATION), respectively. These oppositely charged ions are

attracted to each other, forming an ionic bond.

There is a third type of bonding, called METALLIC BONDING. As the name implies,

metallic bonding usually occurs in metals, such as copper. A piece of copper metal has a

certain arrangement of copper atoms. The valence electrons of these atoms are free to

move about the piece of metal and are attracted to the positive cores of copper, thus

holding the atoms together.

Examples

Covalent Bonds :HCl

Ionic Bond: NaCl

Metallic Bond: Group of Silver atoms held together by metallic bond

3. What is the diameter of a bucky ball? How many pentagons and hexagons are there in a

bucky ball?

C60 mean ball diameter 6.83 Å

C60 ball outer diameter 10.18 Å

C60 ball inner diameter 3.48 Å

12 pentagons and 20 hexagons are there in a bucky ball

4. Describe two scientific discoveries which were responsible for the growing research

interest in nanotechnology.

In the mid 1980s scientists experimented by vaporizing graphite using a laser. A new

substance was formed.

Scientists knew the substance was carbon, but it wasn’t graphite, diamond, or individual

carbon atoms.

They proposed the formula of the material was C16. But C16 fragment – a flat structure

that does not contain hydrogen

The product obtained in the lab was identified by mass spectrometry. The mass spectrum

of the product is shown below.

The evidence points to the formula C60 (mass 720 amu).

However, the buckyball discovery has led to research on a new class of materials called

fullerenes, or buckminsterfullerenes.

Fullerenes are materials with: a three dimensional network of carbon atoms, each atom is

connected to exactly three neighbors, and each atom is bonded by two single bonds and

one double bond (e.g., C82).

5. Give some present and future applications of nanomaterials?

Current Applications

a) Sunscreens and Cosmetics

Nanosized titanium dioxide and zinc oxide are currently used in some sunscreens, as they

absorb and reflect ultraviolet (UV) rays and yet are transparent to visible light and so are more

appealing to the consumer. Nanosized iron oxide is present in some lipsticks as a pigment.

b) Composites

An important use of nanoparticles and nanotubes is in composites, materials that combine

one or more separate components and which are designed to exhibit overall the best properties of

each component. This multi-functionality applies not only to mechanical properties, but extends

to optical, electrical and magnetic ones. Currently, carbon fibres and bundles of multi-walled

CNTs are used in polymers to control or enhance conductivity, with applications such as

antistatic packaging. The use of individual CNTs in composites is a potential long-term

application.

C) Coatings and Surfaces

Coatings with thickness controlled at the nano- or atomic scale have been in routine

production for some time, for example in optoelectonic devices, or in catalytically active and

chemically functionalized surfaces. Recently developed applications include the self-cleaning

window, which is coated in highly activated titanium dioxide, engineered to be highly

hydrophobic (water repellent) and antibacterial, and coatings based on nanoparticulate oxides

that catalytically destroy chemical agents.

Wear and scratch-resistant hard coatings are significantly improved by nanoscale

intermediate layers (or multilayers) between the hard outer layer and the substrate material. The

intermediate layers give good bonding and graded matching of elastic and thermal properties,

thus improving adhesion. A range of enhanced textiles, such as breathable, waterproof and

stainresistant fabrics, have been enabled by the improved control of porosity at the nanoscale and

surface roughness in a variety of polymers and inorganics.

d) Tougher and Harder Cutting Tools

Cutting tools made of nanocrystalline materials, such as tungsten carbide, tantalum

carbide and titanium carbide, are more wear and erosion-resistant, and last longer than their

conventional (large-grained) counterparts. They are finding applications in the drills used to bore

holes in circuit boards.

Short-term Applications (next 5 years)

a) Paints

Incorporating nanoparticles in paints could improve their performance, for example by

making them lighter and giving them different properties. Other novel, and more long-term,

applications for nanoparticles might lie in paints that change colour in response to change in

temperature or chemical environment, or paints that have reduced infra-red absorptivity and so

reduce heat loss.

b) Remediation

The potential of nanoparticles to react with pollutants in soil and groundwater and

transform them into harmless compounds is being researched. In one pilot study the large surface

area and high surface reactivity of iron nanoparticles were exploited to transform chlorinated

hydrocarbons (some of which are believed to be carcinogens) into less harmful end products in

groundwater. It is also hoped that they could be used to transform heavy metals such as lead and

mercury from bioavailable forms into insoluble forms.

c) Displays

The huge market for large area, high brightness, flat-panel displays, as used in television

screens and computer monitors, is driving the development of some nanomaterials.

Nanocrystalline zinc selenide, zinc sulphide, cadmium sulphide and lead telluride are candidates

for the next generation of light-emitting phosphors. CNTs are being investigated for low voltage

field-emission displays; their strength, sharpness, conductivity and inertness make them

potentially very efficient and long-lasting emitters.

d) Batteries

There is great demand for lightweight, high-energy density batteries. Nanocrystalline

materials are candidates for separator plates in batteries because of their foam-like (aerogel)

structure, which can hold considerably more energy than conventional ones. Nickel–metal

hydride batteries made of nanocrystalline nickel and metal hydrides are envisioned to require

less frequent recharging and to last longer because of their large grain boundary (surface) area.

e) Catalysts

Nanoparticles have a high surface area, and hence provide higher catalytic activity. These

more active and durable catalysts could find early application in cleaning up waste streams. This

will be particularly beneficial if it reduces the demand for platinum-group metals, whose use in

standard catalytic units is starting to emerge as a problem, given the limited availability of these

metals.

Longer-term Applications (next 5-15 years)

a) Carbon Nanotube Composites

CNTs have exceptional mechanical properties, particularly high tensile strength and light

weight. An obvious area of application would be in nanotubereinforced composites, with

performance beyond current carbon-fibre composites.

b) Lubricants

Nanospheres of inorganic materials could be used as lubricants by acting as nanosized

‘ball bearings’. The controlled shape is claimed to make them more durable than conventional

solid lubricants and wear additives. There is a claim that this type of lubricant is effective even

if the metal surfaces are not highly smooth.

c) Magnetic Materials

It has been shown that magnets made of nanocrystalline cobalt grains possess unusual

magnetic properties due to their extremely large grain interface area This could lead to

applications in motors, analytical instruments like magnetic resonance imaging (MRI), used

widely in hospitals, and microsensors. Overall magnetisation, however, is currently limited by

the ability to align the grains’ direction of magnetisation.

Nanoscale-fabricated magnetic materials also have applications in data storage. Devices

such as computer hard disks depend on the ability to magnetize small areas of a spinning disk to

record information. If the area required to record one piece of information can be shrunk in the

nanoscale (and can be written and read reliably), the storage capacity of the disk can be

improved dramatically. In the future, the devices on computer chips which currently operate

using flows of electrons could use the magnetic properties of these electrons, called spin, with

numerous advantages.

d) Medical Implants

Current medical implants, such as orthopaedic implants and heart valves, are made of

titanium and stainless steel alloys, primarily because they are biocompatible. Unfortunately, in

some cases these metal alloys may wear out within the lifetime of the patient. Nanocrystalline

zirconium oxide (zirconia) is hard, wearresistant, bio-corrosion resistant and bio-compatible.

Nanocrystalline silicon carbide is a candidate material for artificial heart valves primarily

because of its low weight, high strength and inertness.

e) Water Purification

Nano-engineered membranes could potentially lead to more energy-efficient water

purification processes, notably in desalination by reverse osmosis.

f) Military Battle Suits

Enhanced nanomaterials form the basis of a state-of- the-art ‘battle suit’ that is likely to

be energy-absorbing materials that will withstand blast waves.

Longer-term are those that incorporate sensors to detect or respond to chemical and biological

weapons (for example, responsive nanopores that ‘close’ upon detection of a biological agent).

There is speculation that developments could include materials which monitor physiology while

a soldier is still on the battlefield, and uniforms with potential medical applications, such as

splints for broken bones.

6. Explain the working of Scanning Tunneling Microscopy with a neat sketch?

The Mechanical components of STM should do three things

Bring tip very close to the sample

Keep it there undisturbed (mechanical and thermal vibration isolation)

Scanning on the sample with nanometer resolution A scanning tunneling microscope (STM) is an instrument for imaging surfaces at the atomic level. For an STM, good resolution is considered to be 0.1 nm lateral resolution and 0.01 nm depth resolution. With this resolution, individual atoms within materials are routinely imaged and manipulated. The STM can be used not only in ultra high vacuum but also in air, water, and various other liquid or gas ambients, and at temperatures ranging from near zero kelvin to a few hundred degrees Celsius.

7. Define carbon nanotube? What are the types of carbon nanotubes? Highlight the

properties of carbon nanotubes?

Carbon nanotubes (CNTs) are allotropes of carbon with a cylindrical nanostructure. Nanotubes have been constructed with length-to-diameter ratio of up to 132,000,000:1, significantly larger than any other material. These cylindrical carbon molecules have novel properties, making them potentially useful in many applications in nanotechnology, electronics, optics, and other fields of materials science, as well as potential uses in architectural fields. They may also have applications in the construction of body armor. They exhibit extraordinary strength and unique electrical properties, and are efficient thermal conductors. Types of carbon nanotubes and related structures Armchair (n,n) Zigzag (n,0) Chiral (n,m)

Properties Strength Carbon nanotubes are the strongest and stiffest materials yet discovered in terms of tensile strength and elastic modulus respectively. This strength results from the covalent sp² bonds formed between the individual carbon atoms. Hardness Standard single walled carbon nanotubes can withstand a pressure up to 24GPa without deformation. Kinetic Multi-walled nanotubes are multiple concentric nanotubes precisely nested within one another. These exhibit a striking telescoping property whereby an inner nanotube core may slide, almost without friction, within its outer nanotube shell, thus creating an atomically perfect linear or rotational bearing. Electrical Band structures computed using tight binding approximation for (6,0) CNT (zigzag, metallic) (10,2) CNT (semiconducting) and (10,10) CNT (armchair, metallic).

Because of the symmetry and unique electronic structure of graphene, the structure of a nanotube strongly affects its electrical properties. For a given (n,m) nanotube, if n = m, the nanotube is metallic; if n − m is a multiple of 3, then the nanotube is semiconducting with a very small band gap, otherwise the nanotube is a moderate semiconductor. Thus all armchair (n = m) nanotubes are metallic, and nanotubes (6,4), (9,1), etc. are semiconducting

8. List the methods for producing carbon nanotubes and explain any one of the

method with a neat sketch?

Synthesis of CNT

Techniques have been developed to produce nanotubes in sizeable quantities, including arc discharge, laser ablation, high pressure carbon monoxide (HiPco), and chemical vapor deposition (CVD). Most of these processes take place in vacuum or with process gases. CVD growth of CNTs can occur in vacuum or at atmospheric pressure. Large quantities of nanotubes can be synthesized by these methods; advances in catalysis and continuous growth processes are making CNTs more commercially viable. Arc discharge Nanotubes were observed in 1991 in the carbon soot of graphite electrodes during an arc discharge, by using a current of 100 amps, that was intended to produce fullerenes. However the first macroscopic production of carbon nanotubes was made in 1992 by two researchers at NEC's Fundamental Research Laboratory. The method used was the same as in 1991. During this process, the carbon contained in the negative electrode sublimates because of the high discharge temperatures. Because nanotubes were initially discovered using this technique, it has been the most widely-used method of nanotube synthesis. The yield for this method is up to 30 percent by weight and it produces both single- and multi-walled nanotubes with lengths of up to 50 micrometers with few structural defects. Laser ablation In the laser ablation process, a pulsed laser vaporizes a graphite target in a high-temperature reactor while an inert gas is bled into the chamber. Nanotubes develop on the cooler surfaces of the reactor as the vaporized carbon condenses. A water-cooled surface may be included in the system to collect the nanotubes. This process was developed by Dr. Richard Smalley and co-workers at Rice University, who at the time of the discovery of carbon nanotubes, were blasting metals with a laser to produce various metal molecules. When they heard of the existence of nanotubes they replaced the metals with graphite to create multi-walled carbon nanotubes. Later that year the team used a composite of graphite and metal catalyst particles (the best yield was from a cobalt and nickel mixture) to synthesize single-walled carbon nanotubes. The laser ablation method yields around 70% and produces primarily single-walled carbon nanotubes with a controllable diameter determined by the reaction temperature. However, it is more expensive than either arc discharge or chemical vapor deposition. Chemical vapor deposition (CVD)

Nanotubes being grown by plasma enhanced chemical vapor deposition During CVD, a substrate is prepared with a layer of metal catalyst particles, most commonly nickel, cobalt, iron, or a combination. The metal nanoparticles can also be produced by other ways, including reduction of oxides or oxides solid solutions. The diameters of the nanotubes that are to be grown are related to the size of the metal particles. This can be controlled by patterned (or masked) deposition of the metal, annealing, or by plasma etching of a metal layer. The substrate is heated to approximately 700°C. To initiate the growth of nanotubes, two gases are bled into the reactor: a process gas (such as ammonia, nitrogen or hydrogen) and a carbon-containing gas (such as acetylene, ethylene, ethanol or methane). Nanotubes grow at the sites of the metal catalyst; the carbon-containing gas is broken apart at the surface of the catalyst particle, and the carbon is transported to the edges of the particle, where it forms the nanotubes. This mechanism is still being studied. The catalyst particles can stay at the tips of the growing nanotube during the growth process, or remain at the nanotube base, depending on the adhesion between the catalyst particle and the substrate. Thermal catalytic decomposition of hydrocarbon has become an active area of research and can be a promising route for the bulk production of CNTs. Fluidised bed reactor is the most widely used reactor for CNT preparation. Scale-up of the reactor is the major challenge. CVD is a common method for the commercial production of carbon nanotubes. For this purpose, the metal nanoparticles are mixed with a catalyst support such as MgO or Al2O3 to increase the surface area for higher yield of the catalytic reaction of the carbon feedstock with the metal particles. One issue in this synthesis route is the removal of the catalyst support via an acid treatment, which sometimes could destroy the original structure of the carbon nanotubes. However, alternative catalyst supports that are soluble in water have proven effective for nanotube growth. If a plasma is generated by the application of a strong electric field during the growth process (plasma enhanced chemical vapor deposition*), then the nanotube growth will follow the direction of the electric field.[60] By adjusting the geometry of the reactor it is possible to synthesize vertically aligned carbon nanotubes[61] (i.e., perpendicular to the substrate), a morphology that has been of interest to researchers interested in the electron emission from nanotubes. Without the plasma, the resulting nanotubes are often randomly oriented. Under certain reaction conditions, even in the absence of a plasma, closely spaced nanotubes will maintain a vertical growth direction resulting in a dense array of tubes resembling a carpet or forest. Of the various means for nanotube synthesis, CVD shows the most promise for industrial-scale deposition, because of its price/unit ratio, and because CVD is capable of growing nanotubes directly on a desired substrate, whereas the nanotubes must be collected in the other growth techniques. The growth sites are controllable by careful deposition of the catalyst. In 2007, a team from Meijo University demonstrated a high-efficiency CVD technique for growing carbon nanotubes from camphor.[62] Researchers at Rice University, until recently led by the late Richard Smalley, have concentrated upon finding methods to produce large, pure amounts of particular types of nanotubes. Their approach grows long fibers from many small seeds cut from a single nanotube; all of the resulting fibers were found to be of the same diameter as the original nanotube and are expected to be of the same type as the original nanotube.

9. Describe the principal and working of Atomic Force Microscopy.

Principle of working:

If the tip was scanned at a constant height, a risk would exist that the tip collides with the surface, causing damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to maintain a constant force between the tip and the sample. Traditionally, the sample is mounted on a piezoelectric tube, that can move the sample in the z direction for maintaining a constant force, and the x and y directions for scanning the sample. Alternatively a 'tripod' configuration of three piezo crystals may be employed, with each responsible for scanning in the x,y and z directions. This eliminates some of the distortion effects seen with a tube scanner. In newer designs, the tip is mounted on a vertical piezo scanner while the sample is being scanned in X and Y using another piezo block. The resulting map of the area z = f(x,y) represents the topography of the sample.

10. Explain the working of non-contact mode Atomic Force Microscopy. What are its

advantages over contact mode AFM.

Non-contact mode

AFM - non-contact mode In this mode, the tip of the cantilever does not contact the sample surface. The cantilever is instead oscillated at a frequency slightly above its resonant frequency where the amplitude of oscillation is typically a few nanometers (<10 nm). The van der Waals forces, which are strongest from 1 nm to 10 nm above the surface, or any other long range force which extends above the surface acts to decrease the resonance frequency of the cantilever. This decrease in resonant frequency combined with the feedback loop system maintains a constant oscillation amplitude or frequency by adjusting the average tip-to-sample distance. Measuring the tip-to-sample distance at each (x,y) data point allows the scanning software to construct a topographic image of the sample surface. Non-contact mode AFM does not suffer from tip or sample degradation effects that are sometimes observed after taking numerous scans with contact AFM. This makes non-contact AFM preferable to contact AFM for measuring soft samples. In the case of rigid samples, contact and non-contact images may look the same. However, if a few monolayers of adsorbed fluid are lying on the surface of a rigid sample, the images may look quite different. An AFM operating in contact mode will penetrate the liquid layer to image the underlying surface, whereas in non-contact mode an AFM will oscillate above the adsorbed fluid layer to image both the liquid and surface. Schemes for dynamic mode operation include frequency modulation and the more common amplitude modulation. In frequency modulation, changes in the oscillation frequency provide information about tip-sample interactions. Frequency can be measured with very high sensitivity and thus the frequency modulation mode allows for the use of very stiff cantilevers. Stiff cantilevers provide stability very close to the surface and, as a result, this technique was the first AFM technique to provide true atomic resolution in ultra-high vacuum conditions.[1] In amplitude modulation, changes in the oscillation amplitude or phase provide the feedback signal for imaging. In amplitude modulation, changes in the phase of oscillation can be used to discriminate between different types of materials on the surface. Amplitude modulation can be operated either in the non-contact or in the intermittent contact regime. In dynamic contact mode, the cantilever is oscillated such that the separation distance between the cantilever tip and the sample surface is modulated. Amplitude modulation has also been used in the non-contact regime to image with atomic resolution by using very stiff cantilevers and small amplitudes in an ultra-high vacuum environment.

11. Explain the difference in bondings in Diamond, Graphite and C-60.

Diamond Each carbon atom in a diamond is covalently bonded to four other carbons in a tetrahedron. These tetrahedrons together form a 3-dimensional network of six-membered carbon rings (similar to cyclohexane), in the chair conformation, allowing for zero bond angle strain. This stable network of covalent bonds and hexagonal rings, is the reason that diamond is so incredibly strong. Graphite Graphite has delocalization of the pi bond electrons above and below the planes of the carbon atoms. These electrons are free to move, so are able to conduct electricity. In diamond, all four outer electrons of each carbon atom are 'localised' between the atoms in covalent bonding. The movement of electrons is restricted and diamond does not conduct an electric current. In graphite, each carbon atom uses only 3 of its 4 outer energy level electrons in covalently bonding to three other carbon atoms in a plane. Each carbon atom contributes one electron to a delocalised system of electrons that is also a part of the chemical bonding. The delocalised electrons are free to move throughout the plane. For this reason, graphite conducts electricity along the planes of carbon atoms, but does not conduct in a direction at right angles to the plane. The structure of C60 is a truncated (T = 3) icosahedron, which resembles a soccer ball of the type made of twenty hexagons and twelve pentagons, with a carbon atom at the vertices of each polygon and a bond along each polygon edge. The van der Waals diameter of a C60 molecule is about 1.1 nanometers (nm).[20] The nucleus to nucleus diameter of a C60 molecule is about 0.71 nm. The C60 molecule has two bond lengths. The 6:6 ring bonds (between two hexagons) can be considered "double bonds" and are shorter than the 6:5 bonds (between a hexagon and a pentagon). Its average bond length is 1.4 angstroms.

12. What is the difference between the density of states of 3D,2D, 1D and 0D.

I don’t know the appropriate answer to this question.

13. Explain in details the concept of density of states.

In solid-state and condensed matter physics, the density of states (DOS) of a system describes the number of states per interval of energy at each energy level that are available to be occupied. Unlike isolated systems, like atoms or molecules in gas phase, the density distributions are not discrete like a spectral density but continuous. A high DOS at a specific energy level means that there are many states available for occupation. A DOS of zero means that no states can be occupied at that energy level. In general a DOS is an average over the space and time domains occupied by the system. Local variations, most often due to distortions of the original system, are often called local density of states (LDOS). If the DOS of a undisturbed system is zero, the LDOS can locally be non-zero due to the presence of a local potential.

I don’t know the appropriate answer to this question.

14. Explain with example the effect of quantum confinement on the band structure of

semiconductors.

I don’t know the appropriate answer to this question.

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