Skip to main content
UploadLanguage (EN)Support
BusinessMobileSocial MediaMarketingTechnologyArt & PhotosCareerDesignEducationPresentations & Public SpeakingGovernment & NonprofitHealthcareInternetLawLeadership & ManagementAutomotiveEngineeringSoftwareRecruiting & HRRetailSalesServicesScienceSmall Business & EntrepreneurshipFoodEnvironmentEconomy & FinanceData & AnalyticsInvestor RelationsSportsSpiritualNews & PoliticsTravelSelf ImprovementReal EstateEntertainment & HumorHealth & MedicineDevices & HardwareLifestyle
udhay roopavath, profile picture
Uploaded byudhay roopavath
PDF, PPTX43,241 views

Optical properties of nanomaterials

The document summarizes optical properties of nanomaterials. It discusses topics like optics, optical properties of materials, thin film interference, luminescence, photonic crystals, photoconductivity, solar cells, and optical properties of quantum wells and quantum dots. In particular, it explains how the size-dependent band gap of quantum dots leads to size-tunable fluorescence colors, making quantum dots useful for applications like biological imaging and white LEDs.

Education◦
Download as PDF, PPTX
Optical properties of nanomaterials PRESENTED BY ROOPAVATH UDAY KIRAN M.Tech 1st year Centre for Nano science and Technology Code : NST 621 Course instructor : Associate professor Dr. A.Subramania .
Brief overview of the talk:  Introduction  Optics  Optical properties  The optical material function  Nanoparticle systems and experimental optical observables  References
Introduction
About 40 years ago, research on nanoparticles and nanoparticle matter restarted. The development of new techniques (laser, ESCA, STM, AFM, SNOM, etc.) and the continuous improvement of existing techniques (vacuum technology, electron microscopy, etc.) has allowed new insights into an old subject – the transition from the atom or molecule to the bulk solid matter • One famous example is the Lycurgus cup • Colloidal gold dispersions of Faraday in the nineteenth century. • Industrial manufacturing of stained glass with colloidal particles was established in the seventeenth century • In the nineteenth and twentieth centuries, colloidal nanoparticles found applications mainly in photography based on silver halide nanocrystals, as color pigments, and in catalysis
Optics Light Light is the form of energy detected by the eye, and at ordinary scales can be treated as a wave. Light waves are part of the electromagnetic spectrum, ranging continuously from very long radio waves, with wavelengths of Gm, to high- energy cosmic rays, with wavelengths of the order of Fm. A light wave moving to the right can be represented by the equation:
The electromagnetic spectrum; the visible region only occupies a small part of the whole, from approximately 400–700 nm.
The interaction of materials and radiation The intensity I of an e-m wave, proportional to the square of its amplitude, is a measure of the energy it carries. When radiation with intensity Io strikes a material, a part IR of it is reflected apart IA absorbed, and a part IT may be transmitted. Conservation of energy requires that
Metals absorb photons, capturing their energy by promoting an electron from the filled part of the conduction band into a higher, empty level. When the electron falls back, a photon is reemitted.
The effect of refractive index on the wavelength of light. The wavelength is compressed in materials with a high refractive index.
How does light get through dielectrics? The reason that radiation of certain wavelengths can enter a dielectric is that its Fermi level lies at the top of the valence band, just below a band gap the conduction band with its vast number of empty levels lies above it. To excite an electron across the gap requires a photon with an energy at least as great as the width of the gap, ΔEgap. Thus radiation with photon energy less than ΔEgap cannot excite electrons; there are no energy states within the gap for the electron to be excited into. The radiation sees the material as transparent, offering no interaction of any sort, so it goes straight through.
Luminescence and phosphors The emission of radiation by solids at relatively low temperatures is called luminescence photoluminescence is light emission brought about by the absorption of high-energy photons, typically ultraviolet. The most widely utilised form of photoluminescence is fluorescence, in which light emission is immediate, taking place via allowed transitions Phosphorescence is similar but is typified by the slow conversion of the exciting energy into light, so that light emission is delayed, often by considerable lengths of time, because the light-emitting transitions are forbidden.
Types of Luminescence
Reflection from a single thin film Monochromatic light travelling through air falling upon a homogeneous thin film of an insulator will be reflected from the top surface to give a reflected ray. The light transmitted into the film will be repeatedly reflected from the bottom surface and the underside of the top surface. At each reflection, some of the light will escape to produce additional reflected and transmitted rays. Because of the difference in the paths taken by the repeatedly reflected rays, the waves will interfere with each other.
Diffraction Diffraction occurs when waves interact with objects having a size similar to the wavelength of the radiation. In general, two regimes have been explored in most detail: diffraction quite close to the object which interacts with the light, called Fresnel diffraction, and diffraction far from the object which interacts with the light, called Fraunhofer diffraction. The result of diffraction is a set of bright and dark fringes, due to constructive and destructive interference, called a diffraction pattern. Diffraction gratings Planar diffraction gratings consist of a set of parallel lines with spacing similar to that of the wavelength of light. A transmission grating has alternating clear and opaque lines and diffraction effects are observed in light transmitted by the clear strips. A reflection grating consists of a set of grooves or blazes and diffraction effects are observed in the light reflected from the patterned surface. The effectiveness of a grating is the same whether light is transmitted through it or reflected from it.
Photonic crystals One common form of a one-dimensional photonic crystal is a stack of transparent layers of alternating refractive indices similar to dielectric mirrors. They are called Bragg stacks they behave like selective mirrors that can pass or reflect specific wavelengths of the incident light. The simplest model is that of a transparent material containing regularly- spaced air void. Two-dimensional photonic band gap crystals can be constructed from a two-dimensional array of voids or particles in a transparent medium, and opal is an example of a three-dimensional photonic band gap crystal. Many animals use natural photonic crystal structures for the production of vivid colours, including iridescence in butterfly wings, beetles and feathers.
Photoconductivity and photovoltaic solar cells If radiation of a suitable wavelength falls on a semiconductor, it will excite electrons across the band gap giving rise to a voltage and a related increase in conductivity. Solids that behave in this way are called photovoltaic materials. The magnitude of this photovoltaic or photoconductive effect is roughly proportional to the light intensity. It is used in light meters, exposure meters and automatic shutters in cameras. The first photographic exposure meters used selenium, cadmium sulphide or silicon. In the case of selenium, the photovoltage is large enough to be measured directly and converted to an exposure value. Cadmium sulphide and silicon need voltage amplification, and these materials require a power source, usually a battery, to give a reading. In these materials a voltage is applied across the semiconductor and illumination levels are measured as an increase in conductivity.
• A p-n junction can act in a similar way to a single piece of semiconductor. However, the control afforded by the junction makes the device, a photodiode, far more flexible. As a result, photodiodes are widely used, especially in solar cells. A solar cell is a specialist large-area p-n junction with a depletion region approximately 500 nm thick. (Solar cells must have a large area, to collect as much sunlight as possible.) In addition, the normal built-in potential that exists across the junction, due to the space charge, is engineered to be high (Figure 13). The junction is not connected to any external power source. Holes and electrons produced in the junction region by sunlight are swept across the depletion region by the high built-in space charge present, the electrons going from the p to n region and the holes from the n to p region. This process, called drift, makes the p region more positive and the n region more negative, produces a photovoltage and causes a photocurrent I to flow.
Solar cells, schematic: (a) sunlight falling on a p-n junction creates electron–hole pairs that are swept into the external circuit by the field in the junction region; (b) operating cells use a thin antireflection coating on the front surface, a thin n-type layer, a junction region near to the front surface and a reflecting layer below the cell to increase efficiency.
Dye sensitized solar cells The method of conversion of sunlight to energy in a conventional solar cell is quite different to that of most importance on the Earth, photosynthesis, where the central reactions are oxidation and reduction. Photoelectrochemical cells, of which dye sensitized solar cells (DSSCs), also called Gratzel cells, are an important example, aim to mimic this process. The task of harvesting the light is left to a sensitizer, which is a dye molecule, and the carrier transport task is allocated to a semiconductor. Because the charge separation takes place in the dye, the purity and defect structure of the semiconductor are not crucial to satisfactory operation.
Dye sensitized solar cell schematic. (a) Sunlight absorbed by the dye liberates an electron into the semiconductor. The dye is regenerated by interaction with an internal redox couple. (b) Energy levels in a cell.
The optical properties of quantum wells • In a quantum well, the electrons and holes occupy electron and hole sub-bands . When electrons in the upper energy levels drop to the lower levels in interband transitions, a photon is emitted. The energy separation of the sub-bands is greater than the energy gap of bulk material, so the photons will be of shorter wavelength than those associated with the bulk semiconductor and are said to be blue-shifted compared with the bulk. • Because the dimensions of the quantum well can be changed, the emission spectrum can be varied or tuned. This feature is called bandgap engineering. Quantum well structures are widely used in LEDs and laser diodes to improve device performance. They do this in a number of ways: by confining electrons and holes in a limited space, so that recombination is more likely, and by guiding the output photons by virtue of the differing refractive indices of the materials. Typical of these device structures is the single quantum well (SQW) structure used in the first green- emitting LEDs. A change in the composition of the SQ active layer allows the colour emission to vary between 450 nm blue to 600 nm yellow.
Interband transitions (schematic) between electron (upper) and hole (lower) sub-bands. Green- emitting single quantum well (SQW) active layer LED (schematic).
•The optical properties of photoluminescent nanoparticles, which behave as quantum dots, have been extensively investigated because they emit fluorescent light that is a precise function of the dimensions of the quantum dot. For example, CdSe quantum dots of radius 2.9 nm emit at approximately 555 nm, of radius 3.4 nm emit at approximately 580 nm, and of radius 4.7 nm emit at approximately 625 nm. •To produce fluorescent light, electrons are excited from the lower band to the upper band with ultraviolet radiation. The electrons in higher energy levels subsequently lose energy by non-radiative transitions to end in the lowest energy level of the upper set. A photon is then emitted as the electron drops to the topmost energy level of the lower set The optical properties of nanoparticles
Applications of quantum dots There are many potential applications for photoluminescent quantum dots, because they constitute minute but very bright lamps that can be activated at will by an ultraviolet or blue light probe. Moreover, the colour output is pure in the sense that the emission spectrum is narrow. Applications include • Biological imaging of processes in living cells, • Production of quantum dot lasers and • white LEDs.
Photoluminescent colours emitted by CdS quantum dots. Quantum dot colours: (a) the change inband structure of a quantum dot as the diameter falls;(b) fluorescence colours of different diameter dots(schematic).
Optical properties of nanomaterials