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TeerthankerMahaveerUniversity, Moradabad

College of Engineering

CT-3 (Even Semester) Examination 2015-16 For IIIrd Year/ VIth Semester (

B.Sc. Physics

) Sub. Name: Int. to Nanoscience&Tech.Max Marks: 50

Subject Code: BAS608 Duration: 01:45 hr. Course/Branch/Section:B.Sc./Physics/IIIrdYr

(First – 15 Min.are for distribution and reading of the paper & paper writing time 1 Hr 30 Min.) Note: Attempt all questions.

Q1: (A): Discuss following –

(a) Nanoparticle based Solar Cells. (b) CNT based Transitor.

or [10x2=20] (B): Discuss following –

(a)Application of nanotubes for nano device fabrications. (b)Dip pen lithography.

Q2 (A):Find the solution of wave function for quantum confinement of nano-particles in 3D. or[15x1=15]

(B): Describe briefly the epitaxial growth method of nanoparticle.

Q3(A):Discuss the PVD and CVD technique for the synthesis of nanostructures. or [15x1=15]

(B):Explain the principle and working of Transmission Electron microscopy and its application to Nanostructures.

1(A) (a) Solar Cells and Nanotechnology

Potential advancements in nanotechnology may open the door to the production of cheaper and slightly more efficient solar cells. Conventional solar cells are called photovoltaic cells. These cells are made out of semiconducting material, usually silicon. When light hits the cells, they absorb energy though photons. This absorbed energy knocks out electrons in the silicon, allowing them to flow.

Conventional solar cells have two main drawbacks: they can only achieve efficiencies around 10% and they are expensive to manufacture. The first drawback, inefficiency, is almost unavoidable with silicon cells. This is because the incoming photons, or light, must have the right energy, called the band gap energy, to knock out an electron.

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Nanotechnology might be able to increase the efficiency of solar cells, but the most promising application of nanotechnology is the reduction of manufacturing cost. The nanorods behave as wires because when they absorb light of a specific wavelength they generate electrons. These electrons flow through the nanorods until they reach the aluminum electrode where they are combined to form a current and are used as electricity. This type of cell is cheaper to manufacture than conventional ones for two main reasons. First, these plastic cells are not made from silicon, which can be very expensive. Second, manufacturing of these cells does not require expensive equipment such as clean rooms or vacuum chambers like conventional silicon based solar cells.

Another potential feature of these solar cells is that the nanorods could be

‘tuned’ to absorb various wavelengths of light. This could significantly increase the efficiency of the solar cell because more of the incident light could be utilized .Improvements such as this could make it possible to manufacture inexpensive solar cells with the same efficiency as current technology.

1(A) (b)Electronicdevices:Nanowires still belong to the experimental world of laboratories. However, they may complement or replace carbon nanotubes in some applications. Some early experiments have shown how they can be used to build the next generation of computing devices.

To create active electronic elements, the first key step was to chemically dope a semiconductor nanowire. This has already been done to individual nanowires to create p-type and n-type, p-n junction semiconductors.

Conducting nanowires offer the possibility of connecting molecular-scale entities in a molecular computer. Dispersions of conducting nanowires in different polymers are being investigated for use as transparent electrodes for flexible flat-screen displays.

Sensing of proteins and chemicals using semiconductor nanowires:

In an analogous way to FET devices in which the modulation of conductance (flow of electrons/holes) in the semiconductor, between the input (source) and the output (drain) terminals, is controlled by electrostatic potential variation (gate-electrode) of the charge carriers in the device conduction channel, the methodology of a Bio/Chem-FET is based on the detection of the local change in charge density, or so-called “field effect”, that characterizes the recognition event between a target molecule and the surface receptor.

While several inorganic semiconducting materials such as Si, Ge, or metal oxides (e.g. In2O3, SnO2, ZnO, etc.) have been used for the preparation of nanowires. Silicon nanowires are usually the material of choice when fabricating nanowire FET-based chemo/biosensors.

1(B) (a)Carbon nanotubes (CNTs) are cylinders of one or more layers of graphene (lattice). Diameters of single-walled carbon nanotubes (SWNTs) and multi-single-walled carbon nanotubes (MWNTs) are typically 0.8 to 2 nm and 5 to 20 nm, respectively, although MWNT diameters can exceed 100 nm. CNT lengths range from less than 100 nm to 0.5 m.the prospect of using carbon nanotubes as building blocks to fabricate three-dimensional macroscopic (>1mm in all three dimensions) all-carbon devices.These 3D all-carbon scaffolds/architectures maybe used for the fabrication of the next generation of energy storage, supercapacitors, field emission transistors, high-performance catalysis, photovoltaics, and biomedical devices and implants.

The potential application in different fields are :

(i) Biological and biomedical research: CNTs exhibit dimensional and chemical compatibility with biomolecules, such as DNA and proteins. CNTs enable fluorescent and photoacoustic imaging, as well as localized heating using near-infrared radiation.SWNT biosensors exhibit large changes in electrical impedance and optical properties, which is typically modulated by adsorption of a target on the CNT surface. Low detection limits and high selectivity require engineering the CNT surface and field effects, capacitance, Raman spectral shifts and photoluminescence for sensor design. Products under

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development include printed test strips for estrogen and progesterone detection, microarrays for DNA and protein detection and sensors for NO2 and cardiac troponin. Similar CNT sensors support food industry, military and environmental applications.

(ii) Composite materials: Because of the high mechanical strength of carbon nanotubes, research is being made into weaving them into clothes to create stab-proof and bulletproof clothing. The nanotubes would effectively stop the bullet from penetrating the body, although the bullet's kinetic energy would likely cause broken bones and internal bleeding.

(iii) Mixtures:For load-bearing applications, CNT powders are mixed with polymers or precursor resins to increase stiffness, strength and toughness. These enhancements depend on CNT diameter, aspect ratio, alignment, dispersion and interfacial interaction. Premixed resins and master batches employ CNT loadings from 0.1 to 20 wt %. Nanoscale stick-slip among CNTs and CNT-polymer contacts can increase material damping, enhancing sporting goods, including tennis racquets, baseball bats and bicycle frames. (iv) Textiles:CNT yarns can be knotted without loss of strength. Coating forest-drawn CNT sheets with

functional powder before inserting twist yields weavable, braidable and sewable yarns containing up to 95 wt % powder. Uses include superconducting wires, battery and fuel cell electrodes and self-cleaning textiles.

(v) Microelectronics: Nanotube-based transistors, also known as carbon nanotube field-effect transistors (CNTFETs), have been made that operate at room temperature and that are capable of digital switching using a single electron.SWNTs are attractive for transistors because of their low electron scattering and their bandgap. SWNTs are compatible with field-effect transistor (FET) architectures and high-k dielectrics.

(vi) Coatings and Films:CNTs can serve as a multifunctional coating material. For example, paint/MWNT mixtures can reduce biofouling of ship hulls by discouraging attachment of algae and barnacles. They are a possible alternative to environmentally hazardous biocide-containing paints. Mixing CNTs into anticorrosion coatings for metals can enhance coating stiffness and strength and provide a path for cathodic protection.

1(B) (b)Dip Pen Nanolithography (DPN) allows surface patterning on scales of under sub 100 nanometers. DPN is the nanotechnology analog of the dip pen (also called the quill pen), where the tip of an atomic force microscope cantilever acts as a "pen," which is coated with a chemical compound or mixture acting as an "ink," and put in contact with a substrate, the "paper."

Molecular inks are typically composed of small molecules that are coated onto a DPN tip and are delivered to the surface through a water meniscus. The deposition rate of a molecular ink is dependent on the diffusion rate of the molecule, which is different for each molecule. The size of the feature is controlled by the tip/surface dwell-time (ranging from milliseconds to seconds) and the size of the water meniscus.

Liquid Inks:Liquid inks can be any material that is liquid at deposition conditions. The liquid deposition properties are determined by the interactions between the liquid and the tip, the liquid and the surface, and the viscosity of the liquid itself.

Examples

 Protein, peptide, and DNA patterning; Hydrogels; Sol gels; Conductive inks; Lipids Applications

1. Nanoscale Sensor Fabrication - Small, high-value sensors that can detect multiple targets 2. Nanoscale Protein Chips- High-density protein arrays with increased sensitivity

3. Cell engineering:DPN is emerging as a powerful research tool for manipulating cells at subcellular resolution. Stem cell differentiation, Subcellular drug delivery, Cell sorting, Surface gradients.

4. Dip-Pen" Nanolithography on Semiconductor Surfaces: DPN can also be apply to semiconductor surfaces, such as silicon and gallium arsenide.

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Unique advantages

 Directed Placement - Directly print various materials onto existing nano and microstructures with nanoscale registry

 Direct Write - Maskless creation of arbitrary patterns with feature resolutions from as small as 50 nm and as large as 10 micrometres[15]

 Biocompatible - Subcellular to nanoscale resolution at ambient deposition conditions  Scalable - Force independent, allowing for parallel depositions[16]

 Common misconceptions

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2 (B): Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate.Theoverlayer is called an epitaxial film or epitaxial layer. Epitaxial films may be grown from gaseous or liquid precursors. Because the substrate acts as a seed crystal, the deposited film may lock into one or more crystallographic orientations with respect to the substrate crystal.

Methods :Epitaxial silicon is usually grown using vapor-phase epitaxy (VPE), a modification of chemical vapor deposition. Molecular-beam and liquid-phase epitaxy (MBE and LPE) are also used, mainly for compound semiconductors. Solid-phase epitaxy is used primarily for crystal-damage healing.

Vapor-phase: Silicon is most commonly deposited by doping with silicon tetrachloride and hydrogen at approximately 1200 °C:

SiCl4(g) + 2H2(g) ↔ Si(s) + 4HCl(g)

This reaction is reversible, and the growth rate depends strongly upon the proportion of the two source gases. Growth rates above 2 micrometres per minute produce polycrystalline silicon, and negative growth rates (etching)

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may occur if too much hydrogen chloride byproduct is present. (In fact, hydrogen chloride may be added intentionally to etch the wafer.) An additional etching reaction competes with the deposition reaction:

SiCl4(g) + Si(s) ↔ 2SiCl2(g)

Silicon VPE may also use silane, dichlorosilane, and trichlorosilane source gases. For instance, the silane reaction occurs at 650 °C in this way:

SiH4 → Si + 2H2

This reaction does not inadvertently etch the wafer, and takes place at lower temperatures than deposition from silicon tetrachloride. However, it will form a polycrystalline film unless tightly controlled, and it allows oxidizing species that leak into the reactor to contaminate the epitaxial layer with unwanted compounds such as silicon dioxide.

Liquid-phase: Liquid phase epitaxy (LPE) is a method to grow semiconductor crystal layers from the melt on solid substrates. This happens at temperatures well below the melting point of the deposited semiconductor. The semiconductor is dissolved in the melt of another material. At conditions that are close to the equilibrium between dissolution and deposition, the deposition of the semiconductor crystal on the substrate is relatively fast and uniform. The most used substrate is indium phosphide (InP). Other substrates like glass or ceramic can be applied for special applications. To facilitate nucleation, and to avoid tension in the grown layer the thermal expansion coefficient of substrate and grown layer should be similar.

Solid-phase: Solid Phase Epitaxy (SPE) is a transition between the amorphous and crystalline phases of a material. It is usually done by first depositing a film of amorphous material on a crystalline substrate. The substrate is then heated to crystallize the film. The single crystal substrate serves as a template for crystal growth. The annealing step used to recrystallize or heal silicon layers amorphized during ion implantation is also considered one type of Solid Phase Epitaxy. The Impurity segregation and redistribution at the growing crystal-amorphous layer interface during this process is used to incorporate low-solubility dopants in metals and Silicon.[4]

Molecular-beam epitaxy:In molecular beam epitaxy (MBE), a source material is heated to produce an evaporated beam of particles. These particles travel through a very high vacuum (10−8 Pa; practically free space) to the substrate, where they condense. MBE has lower throughput than other forms of epitaxy. This technique is widely used for growing periodic groupsIII, IV, and V semiconductor crystals.

3(A): Particle precipitation aided CVD:The CVD reaction conditions are so set that particles form by condensation in the gas phase and collect onto a substrate, which is kept under a different condition that allows heterogeneous nucleation. By this method both nanoparticles and particulate films can be prepared. An example of this method has been used to form nanomaterialseg. SnO2, by a method called pyrosol deposition process, where clusters of tin hydroxide are transformed into small aerosol droplets, following which they are reacted onto a heated glass substrate.

CVD technique can be achieved by taking a Process source in the gas phase and using an energy source, such as plasma or a resistively

heated coil, to transfer energy to a gaseous molecule. The CVD process uses hydrocarbons as the carbon sources including methane, carbon monoxide and acetylene. CVD involves the reaction or thermal decomposition of gas phase species at elevated temperatures (~500 -1000 oC). The advantages of the CVD process were low power input, lower temperature range, relatively high purity and, most importantly, possibility to scale up the process.

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Physical Vapour Deposition: Thermal evaporation is one of the simplest and most popular synthesis methods. The basic process of this method is sublimating source material(s) in powder form at high temperature, and a subsequent deposition of the vapor in a certain temperature region to form desired nanostructures.

A typical experimental system is shown in Figure. The synthesis is performed in an alumina or quartz tube, which is located in a horizontal tube furnace. High purity

oxide powders contained in an alumina boat are loaded in the middle of the furnace, the highest temperature region. The substrates for collecting the desired nanostructures are usually placed down-stream following the carrier gas. Cooling water flows inside the cover caps to achieve a reasonable temperature gradient in the tube.

During the experiments, the system is first pumped down to around 10-2Torr. Then the furnace is turned on to heat the tube to the reaction temperature at a specific heating rate. An inert carrying gas, such as argon or nitrogen, is then introduced into the system at a constant flow rate to bring the pressure in the tube back to 200-500 Torr (different pressures are required by different source materials and final deposited nanostructures). The reaction temperature and pressure are held for a certain period of time to vaporize the source material and achieve a reasonable amount of deposition.

Source materials can be vaporized at the high temperature and low pressure condition. The vapor is then carried by the innert carrying gas down to the lower temperature region, where the vapor gradually becomes supersaturated. Once it reaches the substrate, nucleation and growth of nanostructures will occur. The growth is terminated when the furnace is turned off. The system is then cooled down to room temperature with flowing inert gas.

3 (B): Transmission Electron Microscopy (TEM) :The transmission electron microscope is a very powerful tool for material science. A high energy beam of electrons is shone through a very thin sample, and the interactions between the electrons and the atoms can be used to observe features such as the crystal structure and features in the structure like dislocations and grain boundaries. Chemical analysis can also be performed. High resolution can be used to analyze the quality, shape, size and density of quantum wells, wires and dots.

The TEM operates on the same basic principles as the light microscope but uses electrons instead of light. Because the wavelength of electrons is much smaller than that of light, the optimal resolution attainable for TEM images is many orders of magnitude better than that from a light microscope. Thus, TEMs can reveal the finest details of internal structure - in some cases as small as individual atoms.

Imaging: The beam of electrons from the electron gun is focused into a small, thin, coherent beam by the use of the condenser lens. This beam is restricted by the condenser aperture, which

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excludes high angle electrons. The beam then strikes the specimen and parts of it are transmitted depending upon the thickness and electron transparency of the specimen. This transmitted portion is focused by the objective lens into an image on phosphor screen or charge coupled device (CCD) camera. Optional objective apertures can be used to enhance the contrast by blocking out high-angle diffracted electrons. The image then passed down the column through the intermediate and projector lenses, is enlarged all the way.

The image strikes the phosphor screen and light is generated, allowing the user to see the image. The darker areas of the image represent those areas of the sample that fewer electrons are transmitted through while the lighter areas of the image represent those areas of the sample that more electrons were transmitted through.

Diffraction: Fig2. shows a simple sketch of the path of a beam of electrons in a TEM from just above the specimen and down the column to the phosphor screen. As the electrons pass through the sample, they are scattered by the electrostatic potential set up by the constituent elements in the specimen. After passing through the specimen they pass through the electromagnetic objective lens which focuses all the electrons scattered from one point of the specimen into one point in the image plane.

References

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