What is a quantum dot? • Nanocrystals • 2-10 nm diameter • semiconductors What is a quantum dot? • Exciton Bohr Radius • Discrete electron energy levels • Quantum confinement Motivation • Semiconducting nanocrystals are significant due to; strong size dependent optical properties (quantum confinement) • applications solar cells Terahertz gap 1 THz = 300 µm = 33 cm-1 = 4.1 meV Time domain terahertz Spectrometer The pulse width = ΔtFWHM/√2 = 17.6±0.5 fs (A Gaussian pulse is assumed) Terahertz Signal To obtain the response of the sample to the THz radiation 2 measurements are made •THz electric field transmitted through the empty cell •THz electric field transmitted Fourier Transform through the sample cell Terahertz signal Doping • Intentionally adding impurities to change electrical and optical properties • Add free electrons to conduction band or free holes in valence band • Tin and Indium dopants Free carrier Absorption in Quantum Dots Purification and sample preparation of quantum dots Experimental procedure & Data analysis time domain: frequency domain: ~ E (t ) E ( ) ~ T ( ) exp( i ( )) E0 (t ) E0 ( ) Power transmittance Relative phase √T(ω), Φ(ω) Complex refractive index (nr(ω) + i.nim(ω)) No Kramer-Kronig analysis!!! Changes upon charging large quantum dot: Intrinsic Imaginary Dielectric constant The frequency dependent complex dielectric constants determined by experimentally obtained • Frequency dependent absorbance and refractive index. The complex dielectric constant = (nr(î) + ini(î))2 2.0 8 3.2 nm uncharged 6.3 nm uncharged 1.5 3.2 nm charged 6 6.3 nm charged 1.0 im() 4 im() 0.5 2 0.0 0 -0.5 2 3 4 5 6 7 2 3 4 5 6 7 Frequency (THz) Frequency (THz) •For the charged samples Frohlich Band diminishes: A broader and weaker band appears •The reason of this is the presence of coupled plasmon-phonon modes Nano Lett., Vol. 7, No. 8, 2007 Results 2.0 indium doped • Surface undoped phonon 1.5 tin doped • Shift of Absorbance resonance of 1.0 tin doped • Agreement with charged 0.5 QDs 0.0 1 2 3 4 5 6 7 8 THz Results 1.34 Un Sn In 1.32 Ref index 1.30 1.28 1.26 2 3 4 5 6 7 8 THz Semiconductor Quantum Dots Justin Galloway 2-26-07 Department of Materials Science & Engineering Outline I. Introduction II. Effective Mass Model III. Reaction Techniques IV. Applications V. Conclusion How Quantum Dots Semiconductor nanoparticles that exhibit quantum confinement (typically less than 10 nm in diameter) Nanoparticle: a microscopic particle of an inorganic material (e.g. CdSe) or organic material (e.g. polymer, virus) with a diameter less than 100 nm More generally, a particle with diameter less than 1000 nm 1. Gaponenko. Optical properties of semiconductor nanocrystals 2. www.dictionary.com Properties Properties of Quantum Dots Compared to Organic Fluorphores? High quantum yield; often 20 times brighter Narrower and more symmetric emission spectra 100-1000 times more stable to photobleaching High resistance to photo-/chemical degradation Tunable wave length range 400-4000 nm CdSe CdTe http://www.sussex.ac.uk/Users/kaf18/QDSpectra.jpg J. Am. Chem. Soc. 2001, 123, 183-184 Excitation in a Semiconductor Excitation The excitation of an electron from the valance band to the conduction band creates an electron hole pair E ECB h=E g h e (CB) h (VB) Creation of an electron hole pair where h is the photon energy EVB semiconductor Band Gap optical detector (energy barrier) E=h exciton: bound electron and hole pair usually associated with an electron trapped in a localized state in the band gap Recombination of Electron Hole Pairs Release Recombination can happen two ways: radiative and non-radiative E ECB recombination processes EVB E band-to-band recombination recombination atinterband trap states ECB (e.g. dopants, impurities) E=h radiative recombination photon EVB non-radiative recombination phonon (lattice vibrations) e (CB) h (VB) h radiative non-radiative recombination recombination Effective Mass Model Model Developed in 1985 By Louis Brus Relates the band gap to particle size of a spherical quantum dot Band gap of spherical particles The average particle size in suspension can be obtained from the absorption onset using the effective mass model where the band gap E* (in eV) can be approximated by: 1 2 2 1 1 1.8e 0.124e 3 1 1 E * E bulk g 2 m m m m 2er 2 me m0 m h m0 40 r 2 40 e 0 h 0 Egbulk - bulk band gap (eV), h - Plank’s constant (h=6.626x10-34 J·s) r - particle radius e - charge on the electron (1.602x10-19 C) me - electron effective mass - relative permittivity mh - hole effective mass 0 - permittivity of free space (8.854 x10-14 F cm-1) m0 - free electron mass (9.110x10-31 kg) Brus, L. E. J. Phys. Chem. 1986, 90, 2555 Term 2 Model The second term on the rhs is consistent with the particle in a box quantum confinement model Adds the quantum localization energy of effective mass me High Electron confinement due to small size alters the effective mass of an electron compared to a bulk material Consider a particle of mass m confined Poten tia l En erg y in a potential well of length L. n = 1, 2, … • For a 3D box: n2 = nx2 + ny2 + nz2 n2 2 2 n2h2 En 2 x 2mL 8mL2 0 L 1 h2 1 1 1.8e2 0.124e 4 1 1 bulk E* E g 2 mem0 mh m0 40r h2 20 2 mem0 mh m0 8r Brus, L. E. J. Phys. Chem. 1986, 90, 2555 Term 3 Model The Coulombic attraction between electrons and holes lowers the energy Accounts for the interaction of a positive hole me+ and a negative electron me- Electrostatic force (N) between two charges (Coulomb’s Law): qq F 1 2 2 Work, w = F·dr 40r Consider an electron (q=e-) and a hole (q=e+) The decrease in energy on bringing a positive r charge to distance r from a negative charge is: e2 e2 E 2 dr 40r 40r 1 h2 1 1 1.8e2 0.124e 4 1 1 bulk E* E g 2 mem0 mh m0 40r h2 20 2 mem0 mh m0 8r Brus, L. E. J. Phys. Chem. 1986, 90, 2555 Term Influences Model The last term is negligibly small Term one, as expected, dominates as the radius is decreased Modulus Energy (eV) 1 term 1 term 2 term 3 0 0 5 10 d (nm) Conclusion: Control over the particle’s fluorescence is possible by adjusting the radius of the particle Quantum Confinement of ZnO & TiO2 Model ZnO has small effective masses quantum effects can be observed for relatively large particle sizes Confinement effects are observed for particle sizes <~8 nm TiO2 has large effective masses quantum effects are nearly unobservable 4 TiO2 ZnO Eg (eV) Eg (eV) 4 3 3 400 400 on se t (nm) on se t (nm) 350 350 300 300 250 250 0 5 10 0 5 10 d (nm) d (nm) Formation of Nanoparticles The Varying methods for the synthesis of Making nanoparticles Synthesis technique is a function of the material, desired size, quantity and quality of dispersion Synthesis Techniques • Vapor phase (molecular beams, flame synthesis etc… • Solution phase synthesis Semiconductor Nanoparticles •Aqueous Solution II-VI: CdS, CdSe, PbS, ZnS •Nonaqueous Solution III-V: InP, InAs MO: TiO2, ZnO, Fe2O3, PbO, Y2O3 Semiconductor Nanoparticles Synthesis: Typically occurs by the rapid reduction of organmetallic precusors in hot organics with surfactants some examples of in vitro imaging with QDs (http://www.evidenttech.com/) Nucleation and Growth The Making Figure 1. (A) Cartoon depicting the stages of nucleation and growth for the preparation of monodisperse NCs in the framework of the La Mer model. As NCs grow with time, a size series of NCs may be isolated by periodically removing aliquots from the reaction vessel. (B) Representation of the simple synthetic apparatus employed in the preparation of monodisperse NC samples. Horizontal dashed lines represent the critical concentration for nucleation and the saturation concentration C. B. Murray, C. R. Kagan, and M. G. Bawendi, Annu. Rev. Mater. Sci. 30, 545, 2000. Capping Quantum Dots The Due to the extremely high surface area of a Making nanoparticle there is a high quantity of “dangling bonds” Adding a capping agent consisting of a higher band gap energy semiconductor (or smaller) can eliminate dangling bonds and drastically increase Quantum Yield With the addition of CdS/ZnS the Quantum Yield can be increased from ~5% to 55% Synthesis typically consisted of lower concentrated of precursors injected at lower temperatures at slow speeds Shinae, J. Nanotechnology. 2006, 17, 3892 Quantum Dot Images The Quantum dot images prepared in the Searson Lab using Making CdO and TOPSe with a rapid injection 770000x 560000x 455000x Quantum Dot Ligands Provide new Insight Application into erbB/HER receptor – Mediated Signal Transduction Used biotinylated EGF bound to commercial quantum dots QD’s Studied in vitro microscopy the binding of EGF to erbB1 and erbB1 interacts with erbB2 and erbB3 Conclude that QD-ligands are a vital reagent for in vivo studies of signaling pathways – Discovered a novel retrograde transport mechanism Dynamics of endosomal fusion A431 cell expressing erbB3-mCitrine Nat. Biotechnol. 2004, 22; 198-203 Multiplexed Toxin Analysis Using Four Colors Application of Quantum Dot Fluororeagents Demonstrated multiplexed assays for toxins in the same well QD’s Four analyte detection was shown at 1000 and 30 ng/mL for each toxin At high concentrations all four toxins can be deciphered and at low concentrations 3 of the 4 Fluoresence data for all 4 toxin assays at high concentrations Cartoon of assay Anal. Chem. 2004, 76; 684-688 Application Quantum Dot Imaging QDs with antibodies to human prostate-specific membrane antigen indicate murine tumors developed from human prostate cells QD’s 15 nm CdSe/ZnS TOPO/Polymer/PEG/target Gao et al., “In vivo cancer targeting and imaging with semiconductor quantum dots,” Nat. Biotechnol. 22, 969 (2004). Biological Magnetic Nanoparticles Particles Nano-sized magnetic particles can be superparamagnetic Widely Studied – Suggested as early as the 1970’s Offers control/manipulation in magnetic field Co has higher magnetization compared to magnetite and maghemite Science 291, 2001; 2115-2117. J. Phys. D: Appl. Phys. 36, 2003; 167-181. An Attractive Biological Tool Application Magnetic Nanoparticles: Inner Ear Targeted Molecule Delivery and Middle Ear Implant SNP controlled by magnets while transporting a Magnetic payload Particles Studies included in vitro and in vivo on rats, guinea pigs and human cadavers Demonstrated magnetic gradients can enhance drug delivery Perilymphatic fluid from the cochlea of magnet-exposed temporal bone Perilymphatic fluid samples from animals exposed to magnetic forces Audiol Neurotol 2006; 11: 123-133 Magnetic Composite with A Novel Structure for Active Sensing in Living cells Quantum Dot ① Cobalt core : active manipulation diameter : ~10 nm What superparamagnetic NPs is Co MQD ? → manipulated or positioned by an external field without aggregation in CdSe the absence of an external field ZnS Silica ② CdSe shell : imaging with fluorescence thickness : 3-5 nm visible fluorescence (~450 – 700 nm) ④ Silica shell : bio-compatibility & ability to tune the band gap functionalization with specific → by controlling the thickness, able to tune the emission wavelength, i.e., targeting group emission color thickness : ~10 nm ③ ZnS shell : electrical passivation bio-compatible, thickness : 1-2 nm & non-toxic to live cell functions having wider band gap (3.83 eV) stable in aqueous environment than CdSe (1.91 eV) ability to functionalize its surface enhancement of QY with specific targeting group → CdSe (5-10%) CdSe/ZnS (~50%) Conclusions Rap-Up The effective mass model give an excellent approximation of the size dependence of electronic properties of a quantum dot Recent synthesis advances have shown many quantum dot reactions to be robust, cheap, and safe then previously thought Quantum dots offer wide range electronic properties that make them an attractive tool for biological and medical work MQD’s improve afford in vivo manipulation expanded the applicability of quantum dots From an Atom to a Solid Photoemission spectra of negative copper 3d 4s clusters versus number of atoms in the cluster. The highest energy peak corres- ponds to the lowest unoccupied energy level of neutral Cu. Typically, there are two regimes: 1) For < 102 atoms per cluster, the energy levels change rapidly when adding a single atom (e. g. due to spin pairing). 2) For > 102 atoms per cluster, the energy levels change continuously (e. g. due to the electric charging energy (next slide). Energy below the Vacuum level (eV) Energy Levels of Cu Clusters vs. Cluster Radius R Solid Atom ΔE = (E- ER) 1/R (charged sphere) The Band Gap of Silicon Nanoclusters GaAs Bulk Silicon 3 nm : Gap begins to change The Band Gap of Silicon Nanoclusters 3 nm : Gap begins to change Increase of the Band Gap in Small Nanoclusters by Quantum Confinement Conduction Band k2 k1 Gap Valence Band Size Dependent Band Gap in CdSe Nanocrystals The Band Gap of CdSe Size: Nanocrystals Photon Energy vs. Wavelength: h (eV) = 1240 / (nm) Beating the size distribution of quantum dots Quantum dots formed by thin spots in GaAs layers Termination of nanocrystals Critical for their electronic behavior H-terminated Si nanocrystal: Electrons stay inside, passivation, long lifetime Oxyen atom at the surface: Electrons drawn to the oxygen Fluorine at the surface: Complex behavior From Giulia Galli’s group Single Electron Transistor e- e- dot A single electron e- tunnels in two steps from source to drain through the dot. The dot replaces the channel of a normal transistor (below). electrons Designs for Single Electron Transistors Large (≈ m) for operation at liquid He temperature Small (10 nm) for operation around room temperature Nanoparticle attracted electrostatically to the gap between source and drain electrodes. The gate is underneath. Quantum Dots as Artificial Atoms in Two Dimensions * * The elements of this Periodic Table are named after team members from NTT and Delft. Filling electron shells in 2D Magnetic Clusters “Ferric Wheel” Magnetic Nanoclusters in Biology The Holy Grail of Catalysis: Reactions at a Specific Nanoparticle Want this image chemically resolved. Have chemical resolution in micro- spectroscopy via X-ray absorption but insufficient spatial resolution. Fischer-Tropsch process converts coal to fuel using an iron catalyst. Di and De Smit et al., Schlögl Nature (2008) The Oxygen Evolving Complex 4 Mn + 1 Ca Instead of rare metals with 5d or 4d electrons, such as Pt, Rh, Ru, one finds plentiful 3d transition metals in bio - catalysts: Mn, Fe . Nature does it by necessity. Can we do that in artificial photosynthesis ? Biocatalysts = Enzymes Most biocatalysts consist of a protein with a small metal cluster at the active site. The active Fe6Mo center of nitrogenase, Nature’s efficient way of fixing nitrogen.
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