Quantum Dots: Confinement and ApplicationsOutlineRecent History and MotivationQuantum ConfinementConfinement ContinuedWhat is the relevant length scale?Exciton Bohr DiameterSlide 8Experimental Observation of ConfinementOptical AbsorptionThe Blue ShiftBand Gap ComparisonRaman Vibrational SpectroscopyDirection of Raman ShiftPhotoluminescence SpectroscopyPromise from PhotoluminescenceA Brief Look at Biological ApplicationsReferencesQuantum Dots: Confinement and ApplicationsJohn SinclairSolid State IIDr. DagottoSpring 2009OutlineConfinementWhat do we mean?Small dot or Quantum Dot?Experimental EvidenceApplicationsLasersBiologyRecent History and MotivationAdvances in imaging techniques all us to image things at the angstrom levelScanning Tunneling Electron MicroscopesAtomic Force MicroscopyScanning Transmission Electron MicroscopesAFM Image InAsSEM Image of grapheneQuantum Confinement3-DAll carriers act as free carriers in all three directions2-D or Quantum WellsThe carriers act as free carriers in a planeFirst observed in semiconductor systems1-D or Quantum WiresThe carriers are free to move down the direction of the wire0-D or Quantum DotsSystems in which carriers are confined in all directions (no free carriers)Confinement ContinuedSo what if a material is confined in one direction?As the material becomes confined its Density of States changesIn the confined direction you can think of the carriers as particles in boxesWhat is the relevant length scale? Optical ExcitationsOptical excitations should require the band gap In semiconductors excitations exist just below the band gapThe ExcitonThese excitations are bound hole electron pairsBelow the band gap due to binding energyHydrogen like quasi particleHydrogen like energy statesEffective Bohr DiameterExciton 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ΔE ~ 1/M*M* effective mass of excitonSmaller than 3 Bohr Diameter: Strong ConfinementΔE ~ 1/μ*μ* effective mass of hole and electronExciton Bohr DiameterExperimental 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 SpectroscopyOptical 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 wavelengthsThe Blue ShiftThe amount of Blue Shift is a material dependent propertyIt is largest for Ge, but Why?The amount of blue shift scales with the concavity of the band gapParticularly the portion of the band that is important as confinement sets in and the DOS changesBand Gap ComparisonBand gap comparison of Ge and CdTeMust greater concavity of Ge translates to larger blue shiftRaman 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 filmDirection 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Ge dots in a SiO2 matrixPhotoluminescence 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 nmPromise from PhotoluminescencePhotoluminescence spectrum of a 3-layer stack of InP quantum dotsVery narrow absorption should allow for production of great lasersAt present QD lasers only out perform other solid state lasers at low temperatures (below room temperature)Problems arise due to high threshold currents at high temperatureSome QD lasers do not even lase at room temperatureA Brief Look at Biological ApplicationsAttaching ligand molecules and receptors to surface of quantum dots can create new functional form of joined dotsPatterned substrates can cause QDs to form intricate patternsQDs can be used as cellular structure tags with attachment of appropriate ligandsReferencesTracie J. Bukowski, Critical Reviews in Solid State and Materials. Sciences (2002)D. L. Huaker, G. Park and D. G. Deppe, Applied Physics. Journal (1998)S. Hoogland, V. Sukhovatkin, Optics Express. (2006)Teresa Pellegrino, Stefan Kudera and W. J. Parak. small (2005)N. N. Ledentsov, et al., Quantum dot heterostructures: fabrication, properties, lasers. Semiconductors