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11.electronic properties of nanostructured quantum dots

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  • 1. Chemistry and Materials Research www.iiste.orgISSN 2224- 3224 (Print) ISSN 2225- 0956 (Online)Vol 2, No.1, 2012 Electronic Properties of Nanostructured Quantum Dots Syed Bahauddin Alam, Md. Didarul Alam, Tahnia Farheen, Md. Abdullah-Al-Mamun, Md.Rashiduzzaman Bulbul, K M Mohibul Kabir, Farha Sharmin†, Hasan Imtiaz Chowdhury, Md. Abdul Matin Department of EEE, Bangladesh University of Engineering and Technology (BUET), Dhaka †Development Research Network (D.Net) baha_ece@yahoo.comAbstractIn this paper, property analysis of quantum dot cuboid nanocrystals with different nanostructures are shownby simulation results for particular device structure and boundary conditions of Light and dark Transitionsfor the X, Y and Z- Polarized for different structures and so forth. EOM is a systematic method to deriveanalytic expressions for GF Wise (sometimes) extrapolation of perturbation theory. Applied to Andersonmodel, excellent for not-too-strong correlations fair qualitative picture of the Kondo regimeself-consistency improves a lot. Finally from the simulation, it is evident that, the characteristics are almostequivalent for different nanostructures for a particular boundary condition. In this paper electronicproperties of nanostructured quantum dots are analyzed.Keywords: Quantum dot, nanocrystals, nanostructure.1. IntroductionA quantum dot is a semiconductor nanostructure that confines the motion of conduction band electrons,valence band holes, or excitons (pairs of conduction band electrons and valence band holes) in all threespatial directions. A quantum computer is any device for computation that makes direct use of distinctivelyquantum mechanical phenomena, such as superposition and entanglement, to perform operations on data.The quantum properties of particles can be used to represent and structure data and that quantummechanisms can be devised and built to perform operations with these data. Any solid material in the formof a particle with a diameter is comparable to the wavelength of an electron. Quantum Dots is man-madeartificial atoms that confine electrons to a small space. As such they have atomic-like behavior and enablethe study of quantum mechanical effects on a length scale that is around 100 times larger than the pureatomic scale. Quantum dots offer application opportunities in optical sensors, lasers, and advancedelectronic devices for memory and logic. Quantum dots were predicted to exhibit interesting cooperativebehavior in many-dot systems with overlapping wave functions, due to the resulting miniband structure,and also as elements in cellular neural networks .However, no scheme for using discrete quantum dots forcomputing was proposed during this period. These “dots” were not quantum dots in the energy quantizationsense, but rather relied on their ultra small capacitance, which was a consequence of their very small size,to reveal measurable voltage changes with charge variations of only a single electron. Such behavior isclassical, except for tunneling between dots. In confined semiconductor dots, the energy quantization issuperimposed on the Coulombic effects, but is not the primary phenomenon of interest. At the heart of thefluorescence of Quantum dot nanocrystals is the formation of excitons, or Coulomb correlated electron-holepairs. The exciton can be thought of as analogous to the excited state of traditional fluorophores; however,excitons typically have much longer lifetimes (up to ~µseconds), a property that can be advantageous incertain types of "time-gated detection" studies. Yet another distinction arises from the direct, predictablerelationship between the physical size of the quantum dot and the energy of the exciton (therefore, the 1
  • 2. Chemistry and Materials Research www.iiste.orgISSN 2224- 3224 (Print) ISSN 2225- 0956 (Online)Vol 2, No.1, 2012wavelength of emitted fluorescence). This property has been referred to as "tuneability", and is beingwidely exploited in the development of multicolor assays. Quantum dot nanocrystals are also extremelyefficient machines for generating fluorescence; their intrinsic brightness is often many times that observedfor other classes of fluorophores. Another practical benefit of achieving fluorescence without involvingconjugated double-bond systems is that the photostability of Quantum dot nanocrystals is many orders ofmagnitude greater than that associated with traditional fluorescent molecules; this property enableslong-term imaging experiments under conditions that would lead to the photo-induced deterioration of othertypes of fluorophores. The main objective of our experiment was to Study and understand the electronicproperties of coupled quantum dots and to determine the coupling between these dots using vertical electricfields. For that, we have to assemble optical techniques of Photocurrent Measurements andPhotoluminescence. Single quantum dot ensembles are: PC technique successfully applied to single layer ofquantum dots, Stark Shifts and Oscillator Strengths. But we have to sort out for coupled quantum dot.2. Electronic Structure and DimensionsFor Optical Excitation, the characteristics of Exciton are it is bound electron-hole pair and for the Excitesemiconductor, it is creation of electron-hole pair. There are postulates behind these: There is an attractivepotential between electron and hole. In hydrogenic system, mass of hydrogen is greater than electron andfrom Bohr Theory, Binding energy is determined. The excitons confined to the dot are discrete energies andDegree of confinement determined by dot size and Exciton absorption is function-like peaks in absorption.Now for the clarity of the nanostructure of dot, an analysis on comparison between Bits and Quantum bitsare discussed. These are: a Quantum bit can hold a one, a zero, or, crucially, a superposition of these. Aclassical computer has a memory made up of bits, where each bit holds either a one or a zero. The Quantumbits can be in a superposition of all the classically allowed states. The register is described by the phases ofthe numbers can constructively and destructively interferes with one another; this is an important feature forquantum algorithms. For an n quantum bit quantum register, recording the state of the register requires 2ncomplex numbers (the 3-quantum bit register requires 23 = 8 numbers). Consequently, the number ofclassical states encoded in a quantum register grows exponentially with the number of Quantum bits. Forn=300, this is roughly 1090, more states than there are atoms in the observable universe. Electronic structurecan be Formed during epitaxial growth of lattice mismatched materials e.g. InAs on GaAs (7% latticemismatch) and Form due to kinetic and thermodynamic driving forces – energetically more favourable toform nanoscale clusters of InAs. Some general properties are: Perfect crystalline structures, High arealdensity (10-500µm-2), Strong confinement energies (100meV), Lasers (Jth<6Acm-2) in visible and nearinfrared, Quantum Information and Cryptography, For SAQDs - z-axis confinement is generally muchstronger than transverse quantisation x,y (Ez>>Exy). QD states are often approximated as a 2D Harmonicoscillator potential – Fock-Darwin states. Orbital character of QD states similar to atomic systems. Theshells n=1,2,3 - often termed s,p,d,.. in comparison with atomic systems. Dipole allowed optical transitionsDn=0. Single X transitions observable in absorption experiment. QDs should be self-assembled (foreconomic gains, not quite in reach of nano-lithography) for the reasons of semiconductor with smallerbandgap embedded into matrix with large bandgap, just right size, large enough to accommodate an exciton,small enough in all directions for quantum confinement, no structural defects such as dislocations. The keyfactor here is uniformities of size, shape, chemical composition, strain distribution, crystallographic phase,and mutual alignment. For optoelectronics, there is no contacting problem, only problem QD arrayhomogeneity, active medium in lasers, (In,Ga,Al)As based 1.3 µm, far infrared detectors, novel deviceconcepts such as quantum cellular automata, Isolation of a single Quantum Dot, Emission spectroscopy,and Power-dependence reveals the different configurations. 2
  • 3. Chemistry and Materials Research www.iiste.orgISSN 2224- 3224 (Print) ISSN 2225- 0956 (Online)Vol 2, No.1, 20122. Dimensions and ResultsFor a free electron, 3/2kBT = Ñ2k2/2ml ~ 60, for quantum effects: ~10s; in semiconductors, use me* (effectivemass) instead: me */me ~ 1/10, for quantum effects: 100s Þ (10s nm). The number of atoms ~ 103 - 106 .Properties are determined by size of Quantum dots. Small quantum dots, such as colloidal semiconductornanocrystals, can be as small as 2 to 10 nanometers, corresponding to 10 to 50 atoms in diameter and a totalof 100 to 100,000 atoms within the quantum dot volume. At 10 nanometers in diameter, nearly 3 millionquantum dots could be lined up end to end and fit within the width of a human thumb. Quantum wires,which confine the motion of electrons or holes in two spatial directions and allow free propagation in thethird. Quantum wells confine the motion of electrons or holes in one direction and allow free propagation intwo directions. Both have a discrete energy spectrum and bind a small number of electrons. In contrast toatoms, the confinement potential in quantum dots does not necessarily show spherical symmetry. Inaddition, the confined electrons do not move in free space but in the semiconductor host crystal play animportant role for all quantum dot properties. In contrast to atoms, the energy spectrum of a quantum dotcan be engineered by controlling the geometrical size, shape, and the strength of the confinement potential.it is relatively easy to connect quantum dots by tunnel barriers to conducting leads. If now Motiontechnique applied to quantum dot models, according to the Anderson modelHere Generalizations are structured leads (“mesoscopic network”), multilevel dots / multiple dots withcapacitative / tunneling interactions, Spin-orbit interactions and many-body interactions restricted to the dotare essential. We have to treat Anderson model using Perturbation theory (PT) in Γ, tricky to extend intostrong coupling (Kondo) regime, Map to a spin model and do scaling, Numerical Renormalization Group(NRG), accurate low energy physics, integrability condition too restrictive, finite T laborious, Equations ofmotion (EOM).3. ConclusionThe main objective of our experiment was to study and understand the electronic properties of coupledquantum dots and to determine the coupling between these dots using vertical electric fields. In this paperelectronic properties of nanostructured quantum dots are analyzed. Finally from the simulation, it is evidentthat, the characteristics are almost equivalent for different nanostructures for a particular boundarycondition. In this paper electronic properties of nanostructured quantum dots are analyzed.ReferencesSyed Bahauddin Alam et. al, Modeling of Physics of Beta-Decay using Decay Energetics, in AmericanInstitute of Physics (AIP) Proceedings, 2012.Syed Bahauddin Alam et. al Dosimetry Control and Electromagnetic Shielding Analysis, in AmericanInstitute of Physics (AIP) Proceedings, 2012.Syed Bahauddin Alam et. al Transient and Condition Analysis for Gen-4 Nukes for Developing Countries,in American Institute of Physics (AIP) Proceedings, 2012.Syed Bahauddin Alam et al., Methodological Analysis of Bremmstrahlung Emission , published in theWorld Journal of Engineering and Pure and Applied Science, pp: 5-8,Volume: 1, Issue: 1 , Research | 3
  • 4. Chemistry and Materials Research www.iiste.orgISSN 2224- 3224 (Print) ISSN 2225- 0956 (Online)Vol 2, No.1, 2012Reviews | Publications, June 2011.Syed Bahauddin Alam et al., Mathematical Analysis of Poisoning Effect, published in the World Journal ofEngineering and Pure and Applied Science, pp: 15-18, Volume:1, Issue: 1 Research | Reviews |Publications , June 2011.Syed Bahauddin Alam , Md. Nazmus Sakib, Md. Rishad Ahmed, Hussain Mohammed Dipu Kabir, KhaledRedwan, Md. Abdul Matin, Characteristic and Transient Analysis of Gen-4 Nuclear Power via ReactorKinetics and Accelerator Model in 2010 IEEE International Power and Energy Conference, PECON 2010,pp. 113-118, Malaysia, 29 Nov, 2010.Syed Bahauddin Alam, Hussain Mohammed Dipu Kabir, Md. Nazmus Sakib, Celia Shahnaz, ShaikhAnowarul Fattah, EM Shielding, Dosimetry Control and Xe(135)-Sm(149) Poisoning Effect for NuclearWaste Treatment in 2010 IEEE International Power and Energy Conference, PECON 2010, pp.101-106, Malaysia, 29 Nov,2010.Syed Bahauddin Alam, Hussain Mohammed Dipu Kabir, Md. Rishad Ahmed, A B M Rafi Sazzad, CeliaShahnaz, Shaikh Anowarul Fattah, Nuclear Waste Transmutation by Decay Energetics, Compton Imaging,Bremsstrahlung and Nuclei Dynamics in 2010 IEEE International Power and Energy Conference, PECON2010, pp. 107-112, Malaysia, 29 Nov, 2010.Syed Bahauddin Alam, Hussain Mohammed Dipu Kabir, A B M Rafi Sazzad, Khaled Redwan, IshtiaqueAziz, Imranul Kabir Chowdhury, Md. Abdul Matin, Can Gen-4 Nuclear Power and Reactor Technology beSafe and Reliable Future Energy for Developing Countries? In 2010 IEEE International Power and EnergyConference, PECON 2010, pp. 95-100, Malaysia, 29 Nov, 2010.Syed Bahauddin Alam, Md. Nazmus Sakib, Md Sabbir Ahsan, A B M Rafi Sazzad, Imranul KabirChowdhury, Simulation of Bremsstrahlung Production and Emission Process in 2nd InternationalConference on Intelligent Systems, Modelling and Simulation, ISMS2011, Malaysia, Phnom Penh(Cambodia) , 25-27 Jan, 2011.Syed Bahauddin Alam, Md. Nazmus Sakib, Md Sabbir Ahsan, Khaled Redwan, Imranul kabir, Simulationof Beta Transmutation by Decay Energetics in 2nd International Conference on Intelligent Systems,Modelling and Simulation, ISMS2011, Malaysia, Phnom Penh (Cambodia) , 25-27 Jan, 2011.Syed Bahauddin Alam, Md. Nazmus Sakib, A B M Rafi Sazzad, Imranul Kabir in Simulation and Analysisof Advanced Nuclear Reactor and Kinetics Model in 2nd International Conference on Intelligent Systems,Modelling and Simulation, ISMS2011, Malaysia, Phnom Penh (Cambodia) , 25-27 Jan, 2011. 4
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