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Quantum Dots And Their Properties
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- 2. PROJECT TITLE:PROJECT TITLE: ToStudy the OpticalTo Study the Optical Properties ofProperties of CdSCdS Quantum Dots embedded inQuantum Dots embedded in glass matrix and determine theglass matrix and determine the energy band gap and size ofenergy band gap and size of quantum dotsquantum dots
- 3. VISVESVARAYA NATIONAL INSTITUTE OF TECHNOLOGY,NAGPUR DEPARTMENT OF PHYSICS • PROJECT DETAILS • TEAM MEMBERS: Sakshi Deshmukh Sonal Gaikwad Tanvi Kaple •PROJECT GUIDE: Dr. Rupesh .S. Gedam Department of Physics VNIT,Nagpur
- 4. WHAT ARE QUANTUM DOTS? • Quantum dots (QDs) are semiconductor nanocrystals with a core–shell structure and a diameter that typically ranges from 2 to 10 nm. • They are zero dimensional. •They display unique electronic properties intermediate between those of bulk semiconductors and discrete molecules. • They have potential applications in electronic and optoelectronic devices.
- 5. WHO INVENTED QUANTUM DOTS? • Quantum dots were discovered in solids (glass crystals) in 1980 by Russian physicist Alexei Ekimov while working at the Vavilov State Optical Institute. • In late 1982, American chemist Louis E. Brus, then working at Bell Laboratories (and now a professor at Columbia University), discovered the same phenomenon in colloidal solutions. •He discovered that the wavelength of light emitted or absorbed by a quantum dot changed over a period of days as the crystal grew, and concluded that the confinement of electrons was giving the particle quantum properties. • These two scientists shared the Optical Society of America's 2006 R.W. Wood Prize for their pioneering work.
- 6. SYNTHESIS OF QUANTUM DOTS Few methods of synthesis of quantum dots are : • Colloidal Synthesis • Plasma Synthesis •Solvothermal Synthesis • Aqueous and Hydrothermal Synthesis Fig. Cadmium Sulfide Quantum Dots on Cells
- 7. OPTICAL PROPERTIES OF QUANTUM DOTS • The most characteristic property is that their colour is size dependent and thus can be controlled during synthesis. • This arises as a result of quantum confinement effect. •Smaller dots emit higher energy light, that is bluer in colour whereas larger dots emit lower energy red light. Fig. As size increases a red shift is observed
- 8. • QD fluorescence occurs when the excited electron moves from the conduction band to its valence band, emitting a photon with a longer wavelength than the one absorbed (electron–hole recombination process). • QDs can emit light at wavelengths ranging from the ultraviolet (UV) to the infrared (IR). •The properties of QDs include high photostability, high quantum yield and high molar extinction coefficients (~10–100-times those of organic dyes). • They also have narrow symmetrical intense emission at specific wavelengths, ranging from the UV to the IR.
- 9. • The bandgap of QDs exhibits a never observed before direct relationship to their size. • The energy band gap is inversely proportional to the size of quantum dots. •We can vary the energy band gap by changing the size of the quantum dots. • This property of quantum dots makes it possible to use them in a variety of applications.
- 10. QUANTUM CONFINEMENT EFFECT •The quantum confinement effect is observed when the size of the particle is too small to be comparable to the wavelength of the electron. • So as the size of a particle decrease till we a reach a nano scale the decrease in confining dimension makes the energy levels discrete and this increases up the band gap and ultimately the band gap energy also increases. • Since the band gap and wavelength are inversely related to each other the wavelength decrease with decrease in size and the proof is the emission of blue radiation . Fig. Density of electronic states v/s Energy
- 11. APPLICATIONS OF QUANTUM DOTS Some of the applications of quantum dots include: • Solar Cells • Photocatalysts •Lasers • QLED • Computing • Biology QD’S Antibod y Sensor s Battery Cancer Diagnosis
- 12. ADVANTAGES AND LIMITATIONS • A Potential drawback in biological applications is the fact that due to their large physical size, they cannot diffuse across cellular membranes. • Sometimes, a QD may be toxic for the cell and inappropriate for any biological application. •Quantum Dots may blink and become invisible. • Their quite extended lifetime may be a hindrance to certain applications that require QDs to biodegrade immediately after the experiment has been performed. • They are highly efficient in converting short wavelength into longer wavelength of high purity. • The quantum dots are quite stable and can be more easily handled during manufacturing. • They are available in various forms like beads, quantum dust etc. which makes its range of applications wider. • There are multiple methods to develop them easily and cost effectively.
- 13. EXPERIMENTAL WORK • Objective ofthe Work: The aim of this project is to study the effect of size of CdS quantum dots embedded in glass matrix on their energy band gap and optical properties. An attempt was made to determine the energy band gap and size of the quantum dots using absorption spectra. • Synthesis of the sample: Glass system with 3 wt %CdS was prepared by melt quench technique. Fig. Glass prepared by melt-quench technique
- 14. PROCEDURE • Two different types of heat treatments were given to the glass samples for the growth of quantum dots: 1. For same temperature and different time duration 2. For same time duration and different temperatures • The prepared glasses were cut in a suitable shape and polished optically. •The differential thermal analysis of samples was then carried out and the glass transition temperature was noted from DTA analysis. • These glass samples were heat treated with optimized single step heat treatment schedule at a constant temperature but for three different time durations.
- 15. • The glass samples were crushed to powder form to obtain the emission spectra of the glass samples. • Another set of glass samples was given heat treatment for constant time duration and at three different temperatures. •Optical absorption and PL spectra were then recorded . • The energy band gap and size of the quantum dots was determined from the absorption spectra.
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- 17. CONCLUSIONS • We can see that with increasing time duration (temperature remaining constant) as well as in the case of increasing temperature (time remaining constant) the energy band gap is decreasing and consecutively the size of quantum dots is increasing. • We can infer that heat treatment has led to the growth of quantum dots in the glass samples. •The colour of the glass samples changes to yellow from transparent after heat treatment which confirms the growth of QDs. • In the absorption spectra, the cut-off wavelength increases in case of both the types of heat treatment. This confirms the decrease in energy band gap with increase in size of the QDs. • The emission peaks shift towards the longer wavelength side as the time period of heat treatment is increased. • Red shift as well as a decrease in the PL intensity with increase in size was observed in the PL spectra
- 18. FUTURE PROSPECTS • Optical computers could use quantum dots in much the same way that electronic computers use transistors (electronic switching devices)—as the basic components in memory. • Dots can be designed so they accumulate in particular parts of the body and then deliver anti-cancer drugs bound to them. Their big advantage is that they can be targeted at single organs, such as the liver, much more precisely than conventional drugs, so reducing the unpleasant side effects that are characteristic of untargeted, traditional chemotherapy. •QDs have the potential to be much more than just better fluorophores and could open up new frontiers beyond conventional testing strategies.