Nanotechnology and Characterization of Nanomaterials

Transcription

1 Abstract Nanotechnology and Characterization of Nanomaterials Mrs Usha Raghavan Head, Information Technology VPM s Polytechnic, Thane usharagha@gmail.com There s plenty of room at the bottom said Richard Feynman in his first lecture on Nanotechnology way back in Subsequently, it was Eric Drexler, Godfather of Nanotechnology, who popularized and coined the name Nanotechnology. Nanotechnology is tipped to be the next industrial revolution Nanotechnology is a field of applied science focused on the fabrication, Synthesis, characterization and application of materials and devices on the nanoscale. Two main approaches are used in nanotechnology. They are 1) Bottom up approach where materials & devices are built up atom-by-atom 2) Top down where they are synthesized or constructed by removing existing material from larger entities An important aspect of nanotechnology is the increase in the ratio of surface area to volume in nanoscale materials. Materials when reduced to nanoscale show different properties in comparison to the bulk material. The paper will also deal with 1) 1D, 2D and 3D nanomaterials 2) Quantum dots 3) Tools and techniques used for characterizing nanomaterials 4) Applications 5) Risks associated with this technology Keywords: Nanotechnology, quantum dots, Bottom-up, Top down, The author is doing research in nanotechnology under the guidance of Dr Murali Sastry

2 Introduction Nanotechnology is a field of applied science focused on the fabrication, Synthesis, Characterization and application of materials & devices on a nanoscale. Nanomaterials are those which have structured components with at least one dimension less than 100nm.Nano technology is a technology in biology. Physics, chemistry and other scientific fields and involves the manipulation of material in nanoscale Two approaches are used in nanotechnology. One is a bottom-up approach where the materials and devices are built up atom by atom, the other a top- down approach where they are synthesized or constructed by removing the existing material from larger entities. The properties of nanomaterials differ significantly from their bulk counterparts due two factors, one increased relative surface area, and the other the quantum effects. These factors can enhance properties such as reactivity, strength and electrical characteristics. As the particle size decreases, a greater proportion of atoms are found at the surface compared to those inside. Such enhanced properties enable unique applications such as stain resistant cloth, cosmetic lotions. Opaque material becomes transparent and insulators become conductors. Nanotechnology allows scientists to create materials that are stronger, smaller and tougher than ever before History The first mention of concepts in nanotechnology was in "There's Plenty of Room at the Bottom," a talk given by physicist Richard Feynman. The term "nanotechnology" was coined by Professor Norio Taniguchi in a He states "'Nano-technology' mainly consists of the processing of, separation, consolidation, and deformation of materials by one atom or one molecule." In the 1980s the basic idea of this definition was explored in much more depth by Dr. K. Eric Drexler, who promoted the technological penetration of nano-scale phenomena and devices. Types of nanomaterials One-dimensional nanomaterials, such as thin films and engineered surfaces, have been developed and used in electronic device manufacture, chemistry and engineering. In the silicon integratedcircuit industry many devices rely on thin films for their operation, and control of film thicknesses approaching the atomic level is seen. Monolayers are also routinely made and used in chemistry. The formation and properties of these layers are reasonably well understood.advances are being made in the control of the composition and smoothness of surfaces, and the growth of films. Engineered surfaces with tailored properties such as large surface area or specific reactivity are used routinely in a range of applications such as in fuel cells and catalysts. The large surface area provided by nanoparticles, together with their ability to self assemble on a support surface, could be of use in all of these applications. 1D Nano Two dimensional nanomaterials such as tubes and wires have generated considerable interest among the scientific community. Research is going on to study their novel electrical and mechanical properties.

3 Nanowires are linear arrays of dots, formed by self-assembly. Semiconductor nanowires made of silicon, gallium nitride and indium phosphide has demonstrated remarkable optical, electronic and magnetic characteristics (for example, silica nanowires can bend light around very tight corners). Nanowires have potential applications in high-density data storage, either as magnetic read heads or as patterned storage media, and electronic and opto-electronic nanodevices Carbon nanotubes cylinders made of carbon atoms. Their diameter is of few nanometers. Their physical properties like strength, resilience and ductility make them versatile in their applications, 2D Nano carbon Nanotubes 3 dimensional Fullerenes (C60)-can be used as miniature ball bearings to lubricate surfaces, drug delivery vehicles and in electronic circuits 3D Nano - Fullerene If semiconductor particles are made small enough, quantum effects come into play, which limit the energies at which electrons and holes can exist in the particles. As energy is related to wavelength (or colour), this means that the optical properties of the particle can be finely tuned depending on its size. Thus, particles can be made to emit or absorb specific wavelengths (colours) of light, merely by controlling their size. Tools Used for characterizing nanomaterials The various techniques used for studying the characteristics of nanomaterials are: UV- Vis Spectrophotometer X-ray diffraction Transmission Electron Microscope Scanning Electron Microscope Characterization of Nanomaterials

4 Nanotechnology, is a major frontier area of Science and technology to produce materials of nanometer scale. Nanoscience and nanotechnology refer to the control and manipulation of matter at nanometer dimension. The observation of material in the nanoscale can be done using electron, photons, scanning probes, ions, atoms etc. A wide range of technique is available in each of these areas and a systematic application of several tools leads to a complete understanding of the system. Observation is a key to making of new discoveries. It may be done with a probe which may consist of photons, electrons, neutrons, atoms, ions etc. For nanomaterials, the probing light or particle has varying frequency (gamma rays, IR rays). The resulting information can be processed to yield images or spectra which reveal the size, geometric, structural, chemical or physical details of the material. The various tools and techniques used for characterization of nanomaterials are discussed below: UV-Vis spectrophotometer Spectroscopy is the study of matter and its properties by investigating light, sound or particles that are emitted, absorbed or scattered by the matter under investigation. Spectroscopy is the interaction between light and matter. It is used in chemistry for the identification of substances through the spectrum emitted from them or absorbed in them. Different types of spectroscopy use different measurement processes. Absorption spectroscopy uses the range of electro magnetic spectra that a substance absorbs. In atomic absorption spectroscopy, the sample is atomized and the amount of absorption is related to the concentrations of various metals. Emission spectroscopy uses the range of electromagnate spectra in which a substance radiates. The substance first absorbs energy and then radiates this energy as light. Scattering spectroscopy measures certain physical properties by measuring the amount of light that a substance scatters at certain wavelengths, incident angles and polarization angles. Scattering spectroscopy differs from emission spectroscopy. Since the scattering process is faster than the absorption / emission process. UV-Visible Spectroscopy: Ultraviolet and Visible spectroscopy investigates electronic transitions within the molecule or within specific chromophores. UV/Vis give some indication of the multiplicity and arrangement of certain functional groups (Chromophores).

5 A diagram of the components of a typical spectrometer is shown in the above diagram. The functioning of this instrument is relatively straightforward. A beam of light from a visible and/or UV light source is separated into its component wavelengths by a prism or diffraction grating. Each monochromatic (single wavelength) beam in turn is split into two equal intensity beams by a halfmirrored device. One beam, the sample beam, passes through a small transparent container (cuvette) containing a solution of the compound being studied in a transparent solvent. The other beam, the reference, passes through an identical cuvette containing only the solvent. The intensities of these light beams are then measured by electronic detectors and compared. The intensity of the reference beam, which should have suffered little or no light absorption, is defined as I o. The intensity of the sample beam is defined as I. The ration I o /I is called the transmittance T. The absorbance A is given by A = - log (%T) Over a short period of time, the spectrometer automatically scans all the component wavelengths in the manner described. The ultraviolet (UV) region scanned is normally from 200 to 400 nm and the visible portion is from 400 to 800 nm. X-ray diffraction The most effective way of observing the atomic structure of crystals is through the study of diffraction effects, which occur when a beam of x-rays is passed through a crystal. X-ray diffraction is scattering of x-rays by periodic arrangement of atoms forming lattice or crystals. The phenomenon of x-ray diffraction was first discovered and experiments carried out under diffraction by Max Von Laue in If an X-ray beam is directed at a row of equally spaced atoms, each atom becomes a source of scattered waves spreading spherically and reinforce in certain directions to produce the zero-, first-, second-, and higher order diffracted beams.

6 A row of atoms has infinite rotational symmetry along the axes passing through it. Therefore in three dimensions those reinforcement directions of different order are represented with the cones of corresponding order as it is shown in the picture below. Basic Principles: X-ray diffraction method is a powerful tool for investigation orderly arrangements of atoms of molecules thorugh the electromagnetic radiation. The wave length of X-ray is comparable to interatomic distances in crystals and information obtained from wide angle scattering is useful in detecting larger periodicities, which may arise from voids. If the sample is a single crystal it must be placed in all possible orientations usually by rotating or oscillating the couple about one of its axis to achieve desired orientation with respect to the X-ray beam. Otherwise a sample made of a powder of very small crystals may be used. If the minute powdered particles are randomly oriented, all orientations will be included within the sample. Thus the crystalline nature of the sample is determined by carrying out X-ray diffraction of that sample. There are various rays available and according to their energy they can be rated as X-rays, UV rays, visible, IR, microwaves and radio waves. Amongst this, X-ray has minimum wavelength and maximum energy. Every atom in the crystal scatters when an X-ray beam is incident upon it, in all directions Two dimensional array of equally spaced atoms consequently produces scattered waves. The reinforce along the lines of the cross section of two sets of corresponding cones oriented along the coordinate axes. In three-dimesional case the set of the cones oriented along the third coordinate axes causes the reinforcement of scattered waves (constructive interference) to occur at certain locations. Those locations are the points of cross section of all three sets of cones, oriented along three coordinate axes of the crystal. Scanning Electron Microscope In light microscopy, a specimen is viewed through a series of lenses that magnify the visible-light image. However, the scanning electron microscope (SEM) does not actually view a true image of the specimen, but rather produces an electronic map of the specimen that is displayed on a cathode ray tube (CRT). The SEM has a secondary electron detector. The signal produced by the secondary electrons is detected and sent to a CRT image. The scan rate for the electron beam can be increased so that a virtual 3-D image of the specimen can be viewed. The image can also be captured by standard photography.

7 The figure above shows a schematic for a generic SEM. Electrons from a filament in an electron gun are beamed at the specimen in a vaccum chamber. The beam forms a line that continuously sweeps across the specimen at high speed. This irradiates the specimen which in turn produces a signal in the form of either x-ray fluorescence, secondary or backscattered electrons. By changing the width (w) of the electron beam, the magnification (M) can be changed where M= W/w And W is the width of the CRT, Since W is constant, the magnification can be increased by decreasing w. The instrument can be simplified into three major sections: a) electron-optical column : b) vacuum system and c) electronics and display system. A tungsten filament is heated to 2700 K. which produces electrons that are accelerated towards the anode disc. Electrostatic shaping of the electron beam under vacuum gives a beam diameter of about 50 µm. Ultimate performance of the SEM is mainly limited by the diameter of the beam and hence two lenses and condensers demagnify the beam to around 5nm. The scanning coils deflect this beam and sweep it over the specimen surface. A cathode-ray display tube is scanned synchronously with the electron beam. The brightness of the display tube is modulated by the signal, which arises from the intraction of the beam with surface element which is probed. The strength of this signal is translated into image contrast. Secondary electrons, which the beam probe liberates from the specimen surface are collected and used as the contrast signal. The yield of collected electrons depends on the nature of the specimen surface and on its inclination with respect to the probing beam. Consequently, one obtains pictures with a high perspective appearance. A beam of electrons is generated in the electron gun, located at the top of the column, which is pictured to the left. This beam is attracted through the anode, condenser lens, and focused as a very fine point on the sample by the objective lens. The scan coils are energized (by varying the voltage produced by the scan generator) and create a magnetic field, which deflects the beam back and forth in a controlled pattern. The varying voltage is also applied to the coils around the neck of the Cathode-ray tube(crt), which produces a pattern of light deflected back and forth on the surface of the CRT. The pattern of deflection of the electron beam is the same as the pattern of deflection of the spot of light on the CRT.

8 Scanning Electron Microscope The electron bam hits the sample, producing secondary electrons from the sample. These electrons are collected by a secondary detector or a backscatter detector. Converted to a voltage, and amplified. The amplified voltage is applied to the grid of the CRT and causes the intensity of the spot of light to change. The image consists of thousands of spots of varying intensity on the face of a CRT that correspond to the topography of the sample. Sample preparation: The specimens examined by SEM must be able to withstand the strong electric currents produced by the electron beam, Samples that do not conduct electricity can be damaged by the charges that can build up. Non-conductive specimens must first be coated with a thin layer of conductive material. This coating is accomplished using a sputterer. A sputter coater produces a nanometer thickness of conductive material on the surface through a cold plasma process that retains the contours of the specimen. Since most biological samples are non-conductive, they must be sputter coated. However, many of these samples also need additional treatment prior to sputter coating to prevent the cells from collapsing under the intense electron beam. The types of treatments vary according to the specimens. This usually involves a process that fixes the components of the specimen. For example, gluteraldhyde can be used to crosslink the proteins to form rigid polymers. The tissues then need to be rinsed in a buffer followed by dehydrated. Dehydration can be as simple as adding ethanol or more complicated using freeze drying or critical point evaporation. Once the specimen is fixed, it is then glued to a sample holder or post. The post is placed into the sputter coater until a thin layer of gold is applied to the surface. The specimen is then placed in the SEM vacuum chamber and the electron gun is switched on. An Energy Dispersive X-ray Analysis Spectroscope (EDAX) attachment to the SEM helps in determining the elemental constitution of the sample. EDAX is a chemical microanalysis technique performed in conjunction with a SEM. The technique utilizes X-rays that are emitted from the sample, during bombardment of the electron beam of the SEM, to characterize the elemental composition of the analyzed volume. Features or phases as small as 1 µ m can be analyzed. Transmission Electron Microscopy One of the typical characters of nanophase materials is the small particle size. The direct imaging of nanoparticles is only possible using TEM and Scanning probe Microscopy. TEM is unique because it can provide a real space image on the atom distribution in the nanocrystal and on its surface.

9 TEM is a versatile tool that provides: 1) Atomic resolution lattice image. 2) Chemical information at a spatial resolution of 1nm. With finely focused e - probe, the structural characteristics of a single nanoparticle can fully characterized. A TEM consist of 1. Illumination system 2. Specimen stage 3. Objective lens system 4. Magnification system 5. Data recording system 6. Chemical analysis system 1. Illumination system: The electron gun is the heart of illumination system, which uses thermionic emission source or field emission source. The gun gives a high illumination current but the current density and the beam coherence are not as high as that of a field emission. The illumination system also includes a condenser lenses that are important for forming a fine electron probe. 2. Specimen stage is a key for carrying out structure analysis, because it can be used to perform in site observation of phenomenon induced by annealing, e - field or mechanical stress giving the possibility to characterize the physical properties of individual nanostructures. 3. Objective lens is the heart of a TEM determines the limit of image resolution. 4. The magnification system consist of intermediate lenses and projection lenses and it gives magnification up to 1.5 million 5. Data recording system consist of a CCD, allowing quantitative data processing and quantification. 6) Chemical Analysis System is the Energy dispersive x-ray spectroscopy[eds] and e - energyloss spectroscopy[eels]. Both compliment each other to quantify the chemical composition of the specimen. EELS also provide information about the electronic structure of the specimen. Applications The areas that nanotechnology promises to affect are: Pharmacy: It is possible to create biomolecules that carry out Pharmacy in a cell that could release cancer fighting nanoparticles in response to a distress signal from an afflicted cell. Therapeutic Drugs: It is possible to synthesize solid state medicine by producing in nanosize. The high surface area of these small particles allows them to dissolve in bloodstream where larger particles cannot. Tissue Engineering: Nanotechnology can help to reproduce or to repair damaged tissue. Chemistry and environment: Chemical catalysis and filtration techniques are two prominent examples where nanotechnology already plays a role. The synthesis provides novel materials with tailored features and chemical properties Automobile industry: The performance of the automotive coating can be enhanced with nanoproducts that are scratch resistant, water & acid resistant. Nanotyres are more reliable, withreduced wear and tear and are less prone to skidding. Nano battery has twice the storage capacity compared to normal batteries., Energy: The most advanced nanotechnology projects related to energy are storage, conversion, manufacturing improvements by reducing materials and process rates, energy saving e.g. by better thermal insulation, and enhanced renewable energy sources. Environmentally friendly energy systems : An example for an environmentally friendly form of energy is the use of fuel cells powered by hydrogen, which is ideally produced by renewable energies

10 Electronics: Novel Semiconductor devices, opto electronic devices, displays, nanologic, Quantum computers are some of the advances in the field of electronics. Others: Consumer goods, food industry, textiles, sports gadgets will find lot of use of nanomaterials Products that are penetrating the market: A Bangalore based company- Velbionanotech is developing a nanochip which will deliver a drug exactly to the affected area in the body. They are developing a chip which when injected into the body will move towards the kidney and remove stones. Companies like Samsung & LG are selling Acswith nano-coating to ensure fresh air and refrigerators using nano- coating to kill bacteria and keep the frozen food fresh for longer time.arvind Mills, Raymonds and Grasim are manufacturing stain resistant,wrinkle free and odour free garments and fabrics.organic LEDS(OLED) which are much brighter and save more energy compared to LCD used on a TV screen or a mobile phone. Nano acoustic sensors, ultrasonic transducers and inertial sensors are some more developments.it is estimated that nanotechnology market would be worth $1 trillion by Risks associated with nanotechnology Potential risks of nanotechnology can broadly be grouped into three areas: the risk to health and environment from nanoparticles and nanomaterials; the risk posed by molecular manufacturing societal risks. Since nanoparticles are very different from their bulk counterparts, their adverse effects cannot be derived from the known toxicity of the macro-sized material. This poses significant issues for addressing the health and environmental impact of free nanoparticles. Nanoparticles in the atmosphere may be inhaled, swallowed, absorbed through skin which can cause danger to life. It is still not known for sure e if nanoparticles could have undesirable effects on the environment.