Nanotechnology Notes

Nanotechnology Introduction

  • Basic Concepts:

    • Nanotechnology involves manipulating matter at the atomic and molecular levels.

    • Deals with structures and devices in the size range of 1-100 nanometers.

  • Origin and Importance:

    • Richard Feynman's 1959 statement, "There is plenty of room at the bottom," predicted the manipulation of matter at the atomic level.

    • He proposed arranging atoms to create desired structures.

    • Norio Taniguchi coined the term 'nanotechnology' at the University of Tokyo.

  • Definition of Nano:

    • Nano means 10−910^{-9}.

    • A nanometer (nm) is one billionth of a meter (10−910^{-9} m).

Nanomaterials Definition

  • Nanomaterials are materials with structured components having a size less than 100 nm in at least one dimension.

Dimensions

  • Bulk material: Size expressed in 3 dimensions.

  • Planar material: Area expressed in 2 dimensions.

  • Linear material: Length expressed in 1 dimension.

Nanotechnology Defined

  • Nanotechnology involves understanding and controlling matter at dimensions roughly between 1 to 100 nanometers.

  • Unique phenomena at this scale enable novel applications.

  • Encompasses nanoscale science, engineering, and technology.

  • Involves imaging, measuring, modeling, and manipulating matter at the nanoscale.

Nano Scale

  • A nanometer (nm) is one thousand millionth of a meter (10−910^{-9} m).

  • Atoms are extremely small, with diameters ranging from 0.1 to 0.5 nm depending on the element.

    • Example: A carbon atom is approximately 0.15 nm in diameter.

Comparisons

  • Red blood cell: Approximately 7000 nm wide.

  • Water molecule: Almost 0.3 nm across.

  • Human hair: About 80000 nm wide.

Nano Science

  • Nano science is the study of phenomena and manipulation of materials at atomic, molecular, and macromolecular scales.

  • Properties differ significantly from those at a larger scale.

Nanotechnology Defined (Elaborated)

  • Nanotechnology is the design, characterization, production, and application of structures, devices, and systems.

  • Achieved by controlling shape and size at the nanometer scale.

Nanomaterials Defined (Reiterated)

  • Nanomaterials have structured components with a size less than 100 nm in at least one dimension.

Why Nanoparticle Properties Differ

  • Two principal factors cause nanomaterials' properties to differ from other materials:

    1. Quantum confinement

    2. Increase in surface area to volume ratio

Quantum Confinement

  • Quantum confinement effect occurs when the particle size is comparable to the electron's wavelength.

  • Confinement restricts the motion of randomly moving electrons to specific energy levels (discreteness).

  • 'Quantum' reflects the atomic realm of particles.

  • As particle size decreases to the nanoscale, confinement makes energy levels discrete, widening the band gap.

  • Increase in the band gap energy.

Surface Area to Volume Ratio

  • Nanomaterials have a relatively larger surface area compared to the same volume of material in bulk form.

  • Example 1: Sphere

    • Radius 'r'

    • Surface area = 4πr24πr^2

    • Volume = 43πr3\frac{4}{3} πr^3

    • Surface area to volume ratio = 3r\frac{3}{r}

    • As the radius of the sphere decreases, its surface area to volume ratio increases.

  • Example 2: Cube

    • One cube volume: Surface area is 6 m2^2

    • Divided into eight pieces: Surface area becomes 12 m2^2

    • Divided into 27 pieces: Surface area becomes 18 m2^2

    • As the given volume is divided into smaller pieces, the surface area increases.

  • As particle size decreases, a greater portion of atoms are found at the surface compared to those inside.

  • This makes materials more chemically reactive and affects their strength or electrical properties.

Classification of Nanomaterials (Density of States)

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  • Density of States (DOS) function describes the number of states available in a system.

  • Essential for determining carrier concentrations and energy distributions in a semiconductor.

  • Free motion of carriers is limited to two, one, and zero spatial dimensions in semiconductors.

  • Density of states in quantum wells (2D), quantum wires (1D), and quantum dots (0D) must be known.

  • Density of States in 2D:

    • g(E)2D=m∗πℏ2g(E)_{2D} = \frac{m^*}{\pi \hbar^2}

    • Independent of energy.

    • Significant number of available states as the top of the energy gap is reached.

  • Density of States in 1D:

    • g(E){1D} = \frac{m^}{\pi \hbar^2} \sqrt{\frac{2m^}{\pi (E-Ei)}}

  • Density of States in 0D:

    • g(E)<em>0D=2δ(E−E</em>i)g(E)<em>{0D} = 2\delta(E - E</em>i)

    • No free motion is possible.

    • All available states exist only at discrete energies, described by the delta function.

Synthesis of Nanomaterials

  • Nanomaterials are fabricated using different approaches.

  • Two main approaches:

    1. Bottom-up

    2. Top-down

1. Bottom-up Approach

  • Materials and devices are built from molecular components.

  • Assemble chemically using principles of molecular recognition.

  • Example: Sol-Gel method

  • Builds up nanomaterial from the bottom, atom by atom or cluster by cluster.

Sol-Gel Process

  • Example of a bottom-up approach under the chemical method.

  • In solutions, molecules of nanometer size are dispersed and move randomly, hence the solutions are clear.

  • In colloids, the molecules of size ranging from 20µm to 100µm are suspended in a solvent.

  • When mixed with a liquid, colloids look cloudy or even milky.

  • A colloid that suspended in a liquid is called a “Sol”.

  • A suspension that keeps its shape is called a “Gel”.

  • “Sol-Gels” are suspensions of colloids in liquids that keep their shape.

  • “Sol-Gel” formation occurs in different stages like:

    • Hydrolysis

    • Condensation

    • Growth of particles

    • Agglomeration of particles.

  • The rate of hydrolysis and condensation reactions depends on various factors such as pH, temperature, molar reaction, catalyst and process of drying.

  • Under proper conditions, fine Nano particles are produced.

2. Top-down Approach

  • Nano objects are constructed from larger entities without atomic level control.

  • Refers to slicing or successive cutting of Bulk material in to Nano sized particles.

  • Example: Ball Milling method

Ball Milling

  • A method of producing nano materials by mechanical crushing.

  • Mills are equipped with grinding media composed of wolfram carbide or steel.

  • Small balls inside a drum-like cavity are rotated at high speeds and by gravity actions, they settle on a solid layer where they are crushed into nanocrystals.

  • Various types of ball mills:

    1. Attrition ball mill

    2. Planetary ball mill

    3. Vibrating ball mill

    4. Low and high energy ball mills

  • Significant advantage: can be readily implemented commercially.

  • Can be used to make carbon nanotubes and boron nitride nanotubes.

  • Preferred method for preparing metal oxide nano crystals like Cerium (CeO2_2) and Zinc Oxide (ZnO).

Characterization of Nanoparticles

1. Scanning Electron Microscope (SEM)

  • Image produced by scanning the sample with a focused electron beam and detecting secondary and/or backscattered electrons.

  • Electrons and photons are emitted at each beam location and subsequently detected.

  • When transmitted electrons are utilized for imaging, it results in Transmission electron microscopy (TEM).

  • Electrons are used instead of photons, so all lenses are electrostatic / magnetostatic.

Components and Functions

  • Electron gun: Produces a stream of monochromatic electrons.

  • First condenser lens: Condenses the electron stream. Works with the condenser aperture to eliminate high angle electrons from the beam.

  • Second condenser lens: Forms the electrons into a thin, light coherent beam.

  • Objective aperture: Further eliminates high angle electrons from the beam.

  • Scanning coils: A set of coils acting as electrostatic lens scans and sweeps the beam in a grid fashion. The beam dwells on points for a period of time determined by the scan speed (usually in microsecond range).

  • Aperture: Further eliminates high angle electrons from the beam.

  • Interaction with the sample: When the beam strikes the sample, interaction occurs. Before the beam moves to the next dwell point, the various instruments housed to measure various interactions count the number of interactions and display a pixel on a CRT. The intensity of display is determined by the interaction number. More interactions give a brighter pixel.

  • Scanning process: Repeated until the grid scan is finished and then repeated. The entire pattern can be scanned 30 times per second.

Specimen Interaction

  • Specimen interaction results in salient features of an electron microscope.

  • When the energetic electrons strike the sample, various interactions occur.

  • Interactions on the top side of thick or bulk samples result in Scanning Electron Microscope (SEM).

  • Interactions on the bottom side of thin or foil sample result in Transmission Electron Microscope (TEM).

Bulk Specimen Interactions in SEM

  • Backscattered electrons:

    • When an incident electron collides with an atom in the specimen which is nearly normal to the incident path, we get backscattered electron at nearly 1800^0.

    • The intensity of backscattered electrons varies with specimen’s atomic number. Higher atomic number elements.

    • Used to differentiate parts of the specimen that have different average atomic numbers.

  • Secondary electrons:

    • When an incident electron passes very near an atom in the specimen, it may impart some of its energy to the lower energy electron (Usually in the K- shell) resulting in ionization of the electron in the specimen atom.

    • This ionized electron leaves the atom with a very small kinetic energy (~5ev) and is called secondary electrons.

    • Each incident electron can produce several secondary electrons.

    • Since the emitted secondary electrons have low energy, only the secondaries that are very near the surface (< 100nm) can leave the sample.

    • Any change in the topography of the sample changes the yield of the secondary electrons. Image formed collecting secondary electrons gives the topography of the sample.

  • Augar electrons:

    • During the emission of secondary electrons a lower energy electron is released thus leaving a vacancy into inner shell. A higher energy electron from the same atom can fall to the lower energy filling the vacancy.

    • The surplus energy is released by the emission of outer orbit electron. These electrons are called Augar electrons.

    • They have a characteristic energy, unique to each element from which they are emitted. These electrons are collected and sorted according to their energies to give compositional information about the sample.

  • X-rays:

    • When the vacancy due to the emission of secondary electron is filled by the fall of an electron from higher orbit to lower orbit, the difference in energy may be released as x-rays.

    • X-rays thus emitted will have a characteristic energy unique to the element from which it originates.

Applications of SEM

  • Topography: the surface features of an object or "how it looks", its texture, detectable features limited to a few nanometers.

  • Morphology: The shape, size and arrangement of particles making up the object that are lying on the surface of the sample or have been exposed by grinding or chemical etching, detectable features limited to a few nanometers.

  • Composition: The elements and compounds the sample is composed of and their relative ratios, in areas ~ 1 micrometer in diameter.

  • Crystallographic Information: The arrangement of atoms in the specimen and their degree of order, only useful on single crystal particles > 20 micrometers.

Common Semiconductor Applications

  • To view the surface of the device

  • For failure analysis

  • Cross-sectional analysis to determine the device dimensions such as MOSFET channel length or junction depth.

  • On- line inspection of wafer processing production

  • Inspection of integrated-circuits etc.

2. Transmission Electron Microscope (TEM)

Components and Functions

  • Electron gun: Produces a stream of monochromatic electrons.

  • Condenser lenses: Focuses the stream to a small coherent beam.

  • Condenser aperture: Knocks off high angle electrons.

  • Specimen interaction: The beam strikes the specimen.

  • Objective lens: Focuses the transmitted portion into an image.

  • Objective aperture: Enhances the contrast by blocking out high-angle diffracted electrons.

  • Selected area aperture: Enables examining the periodic diffraction of electrons by an ordered arrangement of atoms in the sample.

  • Intermediate and projector lenses: Enlarge the image.

  • Phosphor screen: The beam strikes the phosphor screen and image is formed on the screen. The darker areas of the image represent thicker or denser samples areas since these areas transmit lesser electrons. The brighter areas of the image represent thinner or lesser dense sample areas since these areas transmit more electrons.

Specimen Interactions in TEM

  • Unscattered electrons: These are electrons transmitted through a thin specimen without any interaction occurring inside the specimen. The intensity of transmitted Unscattered electrons is inversely proportional to the thickness of the specimen. Hence thicker areas of the specimen appear darker than the thinner areas.

  • Elastically scattered electrons: These are electrons that are scattered by atoms in the specimen without loss of energy. These scattered electrons are then transmitted through the remaining portions of the specimen. The scattered electrons follow Bragg’s law 2dsinθ=nλ2dsin\theta = n\lambda. Hence by collecting the scattered electrons at different angles, one can get information about the orientation, atomic arrangement and phase present.

  • In elastically scattered electrons: These are electrons that interact with specimen atoms in an inelastic manner, loosing energy. Then they are transmitted through the remaining portions of the specimen.

    • The inelastic loss of energy is characteristic of the elements that have interacted with. These energies are unique to bonding state of each element. Hence this can be used to extract both compositional and bonding information.

TEM Analysis

  • TEM image of Ag Nano particles: The Ag Nano particles are spherical in shape with smooth surface morphology. The diameter of the Nano particles is found to be approximately 15 nm. TEM image also shows that the produced Nano particles are more or less uniform in size and shape.

Applications of TEM

  • Morphology: The size, shape and arrangement of particles as well as their relationship to one another on the scale of atomic diameters.

  • Crystallographic information: The arrangement of atoms in the specimen and their degree of order, detection of atomic-scale defects a few nanometers in diameter.

  • Compositional information: The elements and compounds the sample is composed of and their relative ratios.

3. EDAX or EDX or EDS (Energy Dispersive X-ray analysis)

  • A technique used for identifying the elemental composition of the specimen.

  • EDX analysis system works as an integrated feature of a Scanning Electron Microscope (SEM).

Principle

  • During EDX Analysis, the specimen is bombarded with an electron beam inside the scanning electron microscope.

  • The bombarding electrons collide with the specimen atoms own electrons, knocking some of them off in the process.

  • A position vacated by an ejected inner shell electron is eventually occupied by a higher-energy electron from an outer shell. To be able to do so, however, the transferring outer electron must give up some of its energy by emitting an X-ray.

  • The amount of energy released by the transferring electron depends on which shell it is transferring from, as well as which shell it is transferring to.

  • The atom of every element releases X-rays with unique amounts of energy during the transferring process.

  • By measuring the amounts of energy present in the X-rays being released by a specimen during electron beam bombardment, the identity of the atom from which the X-ray was emitted can be established.

EDX Spectrum

  • The output of an EDX analysis is an EDX spectrum.

  • The EDX spectrum is just a plot of how frequently an X-ray is received for each energy level.

  • An EDX spectrum normally displays peaks corresponding to the energy levels for which the most X-rays had been received.

  • Each of these peaks are unique to an atom, and therefore corresponds to a single element.

  • The higher a peak in a spectrum, the more concentrated the element is in the specimen.

  • An EDX spectrum plot not only identifies the element corresponding to each of its peaks, but the type of X-ray to which it corresponds as well.

  • A peak corresponding to the amount of energy possessed by X-rays emitted by an electron in the L-shell going down to the K-shell is identified as a K-Alpha peak.

  • The peak corresponding to X-rays emitted by M-shell electrons going to the K-shell is identified as a K-Beta peak.

SEM/EDX Analysis

  • A Scanning electron microscope (SEM) can be utilized for high magnification imaging of almost all materials.

  • With SEM in combination of EDX it is also possible to find out the different parts of the sample contains which elements.

  • The SEM/EDAX instruments is a powerful and flexible tool for solving a wide range of product and processing problems for a diverse range of metals and materials.

  • SEM/EDX analysis carried out in many industrial sectors including electronics and semiconductors, pharmaceuticals, plastics and polymers, aerospace, automotives, medical devices, engineering, chemicals, materials and metallurgy.

Applications of EDX

  • Identification of metals and materials

  • Particle contamination (Impurity) identification and elimination

  • Classification of materials

  • Product and process failure and defect analysis

  • Examination of surface morphology

  • Powder morphology, particle size and analysis

  • Cleaning problems and chemical etching

  • Welding and joining technology quality evaluation and failure investigation

  • Paint and coating failure and delaminating (Failure of composite materials) investigation

  • Identification and elimination of corrosion and oxidization problems

  • Contamination or stain investigation

  • Structural analysis

  • Reverse engineering of products and processes