Semiconductor Quantum Dots: CdSe, ZnSe, ZnS, ZnO Group’ members: Trần Phúc Thành Cao Văn Phước Hoàng Văn Tiến
Outline• Introduction – What is semiconductor quantum dots – Why quantum dots – Properties• Synthesis• Applications, challenges, and potentials• Conclusions
Introduction: Image courtesy of Dr. D. Talapin, University of Hamburg
What is Quantumdots?• Quantum dots are semiconductornanocrystals.• They are made of many of the samematerials as ordinary semiconductors (mainlycombinations of transition metals and/ormetalloids).• Unlike ordinary bulk semiconductors, whichare generally macroscopic objects, quantumdots are extremely small, on the order of a fewnanometers. They are very nearly zero-dimensional.
Exciton Bohr Diameter• Material Dependent Parameter – The same size dot of different materials may not both be quantum dots• The Bohr Diameter determines the type of confinement – 3-10 time Bohr Diameter: Weak Confinement – Smaller than 3 Bohr Diameter: Strong Confinement
Experimental Observation of Confinement• Just imaging a small dot is not enough to say it is confined• Optical data allows insight into confinement – Optical Absorption – Raman Vibration Spectroscopy – Photoluminescence Spectroscopy
Optical Absorption• Optical Absorption is a technique that allows one to directly probe the band gap• The band gap edge of a material should be blue shifted if the material is confined• Bukowski et al. present the optical absorption of Ge quantum dots in a SiO2 matrix.• As the dot decreases in size there is a systematic shift of the band gap edge toward shorter wavelengths
Raman Vibrational Spectroscopy• Raman vibrational spectroscopy probes the vibrational modes of a sample using a laser• As the nanocrystal becomes more confined the peak will broaden and shrink• Here we see a peak shift toward the laser line• Various Ge dots of different sizes on an Alumina film
Direction of Raman Shift• Here we see the same broadening and shrinking of the Raman Peak• We see a peak shift away from the laser line• No systematic shift of the Raman line – Shifts toward the laser line are due to confinement – Shifts away from the line are due to lattice tension due to film miss-match
Photoluminescence Spectroscopy• Photoluminescence spectroscopy is a technique to probe the quantum levels of quantum dots• Here we see dots of various size in a quantum well – (a) is quantum well spectrum – (d) is smallest particles 80 nm
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 CdTehttp://www.sussex.ac.uk/Users/kaf18/QDSpectra.jpg J. Am. Chem. Soc. 2001, 123, 183-184
Excitation in a Semiconductor The excitation of an electron from the valance band to the conduction band creates an electron hole pair E ECB hυ E =g − ( ) + ) h↔ BhV υ e C+ ( BCreation of an electron hole pairwhere hν is the photon energy EVB semiconductor Band Gap optical detector (energy barrier) E=hυ 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 Recombination can happen two ways: radiative and non-radiative E ECB recombination processes EVBE band-to-band recom bination recom bination atinterband trap states ECB (e.g. dopants, impurities) E = hυ radiative recombination → photon EVB non-radiative recombination → phonon (lattice vibrations) − ) + ) υ ra d ia tive n o n -ra d ia tive re co m b in a tio n re c o m b in a tio n e C+ ( B h ( BhV→
Effective Mass Model Developed in 1985 By Louis Brus Relates the band gap to particle size of a spherical quantum dotBand gap of spherical particlesThe average particle size in suspension can be obtained from the absorptiononset using the effective mass model where the band gap E* (in eV) can beapproximated by: − π .e 1 h2 1 1 1 03 1 1 2 8 .42e 1 E + + = − − + *b u l k E 2 m hπ h 2 ε 0 h m ε ( 0m m ) m ε 4 2e m g 0 2 e e r 0 m 04 r π ε m0 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 ε - relative permittivitymh - hole effective mass ε0 - 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 1 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 materialConsider a particle of mass m confined P oten tia l E nergyin a potential well of length L. n = 1, 2, … For a 3D box: n2 = nx2 + ny2 + nz2 2 π n h2 2 n h 22 E= n 2 = x 2 L 8 L m m2 0 L − h 1 1 1e . 2 0 2e 1 1 2 1 8 . 44 1E = g lk+ 2 * Eu b + − − + r m 0 m 0 4 ε 0 h ( ε 0 2 e 0 m 0 8 em hm πεr 2 2ε ) m m hm Brus, L. E. J. Phys. Chem. 1986, 90, 2555
Term 2 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 εε 4π 0rConsider an electron (q=e-) and a hole (q=e+)The decrease in energy on bringing a positive rcharge to distance r from a negative charge is: 2 2 e e ∆ =∫ E d= r − 4 ε0 πε r2 4 ε0 πε r − h 1 1 1e . 2 0 2e 1 1 2 1 8 . 44 1 E = g l + 2 * Euk b + − − + r m 0 m 0 4 ε 0 h ( ε 0 2 e 0 m 0 8 em hm πεr 2 2ε ) m m hm Brus, L. E. J. Phys. Chem. 1986, 90, 2555
Term InfluencesThe last term is negligibly smallTerm one, as expected, dominates as the radius is decreased Energy (eV ) Modulus 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 ZnO has small effective masses quantum effects can be observed for relatively large particle sizes Confinement effects are observed for particle sizes <~8 nm ZnO CdSe ZnO λbulk = 365 nm λbulk = 709 nm ε = 8.66 ε = 10.6 Eg (eV) 4 me* = 0.24 me* = 0.13 mh* = 0.59 mh* = 0.45 3 400λ onset (nm) 350 300 250 0 5 10 d (nm)
Fabrication Methods• Goal: to engineer potential energy barriers to confine electrons in 3 dimensions• 3 primary methods – Lithography – Colloidal chemistry – Epitaxy
2. Fabrication of Quantum Dots How to Make Quantum Dots• There are three main ways to confine excitons in semiconductors: – Lithography – Colloidal synthesis – Epitaxy: » Patterned Growth » Self-Organized Growth21
2. Fabrication of Quantum Dots Lithography• Quantum wells are covered with a polymer mask and exposed to an electron or ion beam.• The surface is covered with a thin layer of metal, then cleaned and only the exposed areas keep the metal layer.• Pillars are etched into the entire surface.• Multiple layers are applied this way to build up the properties and size wanted.• Disadvantages: slow, contamination, low density, defect formation.Resulting quantum dot etched in GaAs/AlGaAs superlattice.22 L. Jacak, P. Hawrylak, A. Wojs. Quantum dots fig. 2.2.
2. Fabrication of Quantum Dots Colloidal Synthesis• Immersion of semiconductor microcrystals in glass dielectric matrices.• Taking a silicate glass with 1% semiconducting phase (CdS, CuCl, CdSe, or CuBr).• Heating for several hours at high temperature.⇒ Formation of microcrystals of nearly equal size.• Typically group II-VI materials (e.g. CdS, CdSe)• Size variations (“size dispersion”).23
2. Fabrication of Quantum Dots Epitaxy: Patterned Growth• Semiconducting compounds with a smaller bandgap (GaAs) are grown on the surface of a compoundwith a larger bandgap (AlGaAs).• Growth is restricted by coating it with a masking compound (SiO2) and etching that mask with the shape of the required crystal cell wall shape.• Disadvantage: density of quantum dots limited by mask pattern. L. Jacak, P. Hawrylak, A. Wojs. Quantum dots24 fig 2.7.
2. Fabrication of Quantum Dots Epitaxy: Self-Organized Growth• Uses a large difference in the lattice constants of the substrate and the crystallizing material.• When the crystallized layer is thicker than the critical thickness, there is a strong strain on the layers.• The breakdown results in randomly distributed islets of regular shape and size.• Disadvantages: size and shape fluctuations, ordering. Schematic drawing of lens-shaped self- organized quantum dot. L. Jacak, P. Hawrylak, A. Wojs. Quantum dots25 fig 8.1.
Lithography• Etch pillars in quantum well heterostructures – Quantum well heterostructures give 1D confinement • Mismatch of bandgaps ⇒ potential energy well – Pillars provide confinement in the other 2 dimensions• Electron beam lithography• Disadvantages: Slow, contamination, low density, defect formation A. Scherer and H.G. Craighead. Fabrication of small laterally patterned multiple quantum wells. Appl. Phys. Lett., Nov 1986.
Colloidal Particles• Engineer reactions to precipitate quantum dots from solutions or a host material (e.g. polymer)• In some cases, need to “cap” the surface so the dot remains chemically stable (i.e. bond other molecules on the surface)• Can form “core-shell” structures• Typically group II-VI materials (e.g. CdS, CdSe)• Size variations ( “size dispersion”) CdSe core with ZnS shell QDs Red: bigger dots! Blue: smaller dots! Evident Technologies: http://www.evidenttech.com/products/core_shell_evidots/overview.php Sample papers: Steigerwald et al. Surface derivation and isolation of semiconductor cluster molecules. J. Am. Chem. Soc., 1988.
Epitaxy: Patterned Growth• Growth on patterned substrates – Grow QDs in pyramid-shaped recesses – Recesses formed by selective ion etching – Disadvantage: density of QDs limited by mask pattern T. Fukui et al. GaAs tetrahedral quantum dot structures fabricated using selective area metal organic chemical vapor deposition. Appl. Phys. Lett. May, 1991
Epitaxy: Self-Organized Growth• Self-organized QDs through epitaxial growth strains – Stranski-Krastanov growth mode (use MBE, MOCVD) • Islands formed on wetting layer due to lattice mismatch (size ~10s nm) – Disadvantage: size and shape fluctuations, ordering – Control island initiation • Induce local strain, grow on dislocation, vary growth conditions, combine with patterning AFM images of islands epitaxiall grown on GaAs substrate. (a) InAs islands randomly nucleate. (b) Random distribution of InxGa1xAs ringshaped islands. (c) A 2D lattice of InAs islands on a GaAs substrate. P. Petroff, A. Lorke, and A. Imamoglu. Epitaxially self-assembled quantum dots. Physics Today, May 2001.
Quantum computingA quantum computer is a computer design which uses the principles of quantum physito increase the computational power beyond what is attainable by a traditional compute
The tiny seven-atom quantum dot created by scientists (left), with a close-up of that dot forming an atomic-scale transistor (right) essential for the creation of semiconductorbased quantum computers,which could be faster and provide more memory than conventionaltechnology.
Rules:-By applying small voltages to the leads, the flow of electrons through thequantum dot can be controlled and thereby precise measurements of thespin and other properties therein can be made.-With several entangled quantum dots, or qubits , plus a way of performingoperations, quantum calculations and the computers that would performthem might be possible. Bits :a bit can be defined as a variable or computed quantity that can have only two possible values : 0/1 or on/off… Qubits: -quantum mechanics allows the qubit to be in a superposition of both states at the same time, a property which is fundamental to quantum computing. - a qubit can be 0, 1, or a superposition of both.
Quantum-dot light-emmiting diodes ( QDLED)
-The spectrum of photon emission is narrow, characterized by its fullwidth at half the maximum value-pure and saturated emission colors with narrow bandwidth- emission wavelength is easily tuned by changing the size of thequantum dots
• Advantages : – Low power consumption – Ranger and accuracy color – Brightness: 50~100 times brighter than CRT and LCD displays ~40,000 cd/m2 – QLED screens are said to be twice as power efficient as OLED screens, and offer 30 to 40% improved brightness.• Disadvantages : – Less saturated blue – expensive
Quantum dot solar cell• Quantum dots have bandgaps that are tunable across a wide range of energy levels by changing the quantum dot size.
5–20 layer QD-enhanced solar cells show a net increase in external quantumefficiency (EQE) at IR wavelengths compared to the baseline GaAs
How Can Quantum Dots Improve the Efficiency?• Quantum dots can generate multiple exciton (electron- hole pairs) after collision with one photon.39 39
Spincast quantum dot solar cell roll-to-roll solar cell fabrication.• Results: – -conversion efficiencies up to a record-breaking 7 percent efficiency. – The efficiency of solar cells could be increased to more than 60% from the current limit of just 30%...
Another applications• Photodetector devices• Quantum dots laser• Colloidal quantum dots• Uses in biology:use of QDs as passive labels in sensor applications, gene technology, pathogen and toxin detection…
Challenges and potentials• Potentials : – The composition and very small size of quantum dots (5–8 nm) gives them unique and very stable fluorescent optical properties that are readily tunable by changing their physical composition or size. – Their broad absorption spectra but very narrow emission spectra allows multiplexing of many quantum dots of different colors in the same sample. – quantum dots can be made from many different elements, and come in many different shapes and sizes,• Challenges : – Hard to control the side of dots. – Cytotoxic Effect – Environmental toxicity: • Phytotoxicity • Marine toxicity
conclusion• Quantum dot are the new and inovative perspective on the traditional semiconductor• Quantum dot can be synthesized to be essentially any size, and therefore produce essentially any wavelength of light• Application of Quantum Dots that they are useful in many conditions due to their physiochemical properties Quantum Dots may used in different fields by altering their surface property, internal structures, preparation techniques, coating material etc.• The future looks bright and exciting on all the possible applications of quantum dots
references 1, http://en.wikipedia.org/wiki/Quantum_dot2. http://nature.com3. http://google.com4. Y. Masumoto and T. Takagahara. Semiconductor Quantum Dots: Physics, Spectroscopy, and Applications. New York: Springer-Verlag, 2002.5. Synthesis and Applications of Nanoparticles and Quantum Dots-Richard D. Tilley6. http://www.nanowerk.com7. http://led-professional.com8. http://quantumdotstech.com/9. http://youtube.com………